Make style fixes to conform to ruby style guide

Made style fixes to conform to Ruby style guide.
Added a reference to a community-driven Ruby coding style guide.

ruby.html.markdown

@@ -9,6 +9,7 @@ contributors:
   - ["Nick LaMuro", "https://github.com/NickLaMuro"]
   - ["Marcos Brizeno", "http://www.about.me/marcosbrizeno"]
   - ["Ariel Krakowski", "http://www.learneroo.com"]
+  - ["Dzianis Dashkevich", "https://github.com/dskecse"]
 
 ---
 
@@ -35,7 +36,7 @@ You shouldn't either
 8 - 1 #=> 7
 10 * 2 #=> 20
 35 / 5 #=> 7
-2 ** 5 #=> 32
+2**5 #=> 32
 
 # Arithmetic is just syntactic sugar
 # for calling a method on an object
@@ -78,14 +79,17 @@ false.class #=> FalseClass
 'I am a string'.class #=> String
 "I am a string too".class #=> String
 
-placeholder = "use string interpolation"
+placeholder = 'use string interpolation'
 "I can #{placeholder} when using double quoted strings"
 #=> "I can use string interpolation when using double quoted strings"
 
+# Prefer single quoted strings to double quoted ones where possible
+# Double quoted strings perform additional inner calculations
+
 # Combine strings, but not with numbers
-"hello " + "world"  #=> "hello world"
-"hello " + 3 #=> TypeError: can't convert Fixnum into String
-"hello " + 3.to_s #=> "hello 3"
+'hello ' + 'world'  #=> "hello world"
+'hello ' + 3 #=> TypeError: can't convert Fixnum into String
+'hello ' + 3.to_s #=> "hello 3"
 
 # print to the output
 puts "I'm printing!"
@@ -130,7 +134,7 @@ array = [1, 2, 3, 4, 5] #=> [1, 2, 3, 4, 5]
 
 # Arrays can contain different types of items
 
-[1, "hello", false] #=> [1, "hello", false]
+[1, 'hello', false] #=> [1, "hello", false]
 
 # Arrays can be indexed
 # From the front
@@ -157,7 +161,7 @@ array << 6 #=> [1, 2, 3, 4, 5, 6]
 
 # Hashes are Ruby's primary dictionary with keys/value pairs.
 # Hashes are denoted with curly braces:
-hash = {'color' => 'green', 'number' => 5}
+hash = { 'color' => 'green', 'number' => 5 }
 
 hash.keys #=> ['color', 'number']
 
@@ -170,7 +174,7 @@ hash['nothing here'] #=> nil
 
 # Since Ruby 1.9, there's a special syntax when using symbols as keys:
 
-new_hash = { defcon: 3, action: true}
+new_hash = { defcon: 3, action: true }
 
 new_hash.keys #=> [:defcon, :action]
 
@@ -180,11 +184,11 @@ new_hash.keys #=> [:defcon, :action]
 # Control structures
 
 if true
-  "if statement"
+  'if statement'
 elsif false
-  "else if, optional"
+  'else if, optional'
 else
-  "else, also optional"
+  'else, also optional'
 end
 
 for counter in 1..5
@@ -216,7 +220,7 @@ end
 #=> iteration 5
 
 # You can also surround blocks in curly brackets:
-(1..5).each {|counter| puts "iteration #{counter}"}
+(1..5).each { |counter| puts "iteration #{counter}" }
 
 # The contents of data structures can also be iterated using each.
 array.each do |element|
@@ -241,32 +245,30 @@ grade = 'B'
 
 case grade
 when 'A'
-  puts "Way to go kiddo"
+  puts 'Way to go kiddo'
 when 'B'
-  puts "Better luck next time"
+  puts 'Better luck next time'
 when 'C'
-  puts "You can do better"
+  puts 'You can do better'
 when 'D'
-  puts "Scraping through"
+  puts 'Scraping through'
 when 'F'
-  puts "You failed!"
+  puts 'You failed!'
 else
-  puts "Alternative grading system, eh?"
+  puts 'Alternative grading system, eh?'
 end
-
 #=> "Better luck next time"
 
 # cases can also use ranges
 grade = 82
 case grade
-    when 90..100
-      puts "Hooray!"
-    when 80...90
-      puts "OK job" 
-    else
-      puts "You failed!"
+when 90..100
+  puts 'Hooray!'
+when 80...90
+  puts 'OK job'
+else
+  puts 'You failed!'
 end
-
 #=> "OK job"
 
 
@@ -284,23 +286,23 @@ double 3 #=> 6
 
 double double 3 #=> 12
 
-def sum(x,y)
+def sum(x, y)
   x + y
 end
 
 # Method arguments are separated by a comma
 sum 3, 4 #=> 7
 
-sum sum(3,4), 5 #=> 12
+sum sum(3, 4), 5 #=> 12
 
 # yield
 # All methods have an implicit, optional block parameter
 # it can be called with the 'yield' keyword
 
 def surround
-  puts "{"
+  puts '{'
   yield
-  puts "}"
+  puts '}'
 end
 
 surround { puts 'hello world' }
@@ -313,23 +315,23 @@ surround { puts 'hello world' }
 # You can pass a block to a function
 # "&" marks a reference to a passed block
 def guests(&block)
- block.call "some_argument" 
+  block.call 'some_argument'
 end
 
 # You can pass a list of arguments, which will be converted into an array
 # That's what splat operator ("*") is for
 def guests(*array)
- array.each { |guest| puts "#{guest}" }
+  array.each { |guest| puts guest }
 end
 
 # Define a class with the class keyword
 class Human
 
   # A class variable. It is shared by all instances of this class.
-  @@species = "H. sapiens"
+  @@species = 'H. sapiens'
 
   # Basic initializer
-  def initialize(name, age=0)
+  def initialize(name, age = 0)
     # Assign the argument to the "name" instance variable for the instance
     @name = name
     # If no age given, we will fall back to the default in the arguments list.
@@ -356,20 +358,19 @@ class Human
   # A class method uses self to distinguish from instance methods.
   # It can only be called on the class, not an instance.
   def self.say(msg)
-    puts "#{msg}"
+    puts msg
   end
 
   def species
     @@species
   end
-
 end
 
 
 # Instantiate a class
-jim = Human.new("Jim Halpert")
+jim = Human.new('Jim Halpert')
 
-dwight = Human.new("Dwight K. Schrute")
+dwight = Human.new('Dwight K. Schrute')
 
 # Let's call a couple of methods
 jim.species #=> "H. sapiens"
@@ -380,7 +381,7 @@ dwight.species #=> "H. sapiens"
 dwight.name #=> "Dwight K. Schrute"
 
 # Call the class method
-Human.say("Hi") #=> "Hi"
+Human.say('Hi') #=> "Hi"
 
 # Variable's scopes are defined by the way we name them.
 # Variables that start with $ have global scope
@@ -399,7 +400,7 @@ defined? @@var #=> "class variable"
 Var = "I'm a constant"
 defined? Var #=> "constant"
 
-# Class also is object in ruby. So class can have instance variables.
+# Class is also an object in ruby. So class can have instance variables.
 # Class variable is shared among the class and all of its descendants.
 
 # base class
@@ -415,7 +416,7 @@ class Human
   end
 end
 
-#  derived class
+# derived class
 class Worker < Human
 end
 
@@ -451,8 +452,8 @@ module ModuleExample
   end
 end
 
-# Including modules binds the methods to the object instance
-# Extending modules binds the methods to the class instance
+# Including modules binds their methods to the class instances
+# Extending modules binds their methods to the class itself
 
 class Person
   include ModuleExample
@@ -467,7 +468,7 @@ Person.new.foo # => 'foo'
 Book.foo       # => 'foo'
 Book.new.foo   # => NoMethodError: undefined method `foo'
 
-# Callbacks when including and extending a module are executed
+# Callbacks are executed when including and extending a module
 
 module ConcernExample
   def self.included(base)
@@ -504,5 +505,4 @@ Something.new.qux # => 'qux'
 - [Official Documentation](http://www.ruby-doc.org/core-2.1.1/)
 - [Ruby from other languages](https://www.ruby-lang.org/en/documentation/ruby-from-other-languages/)
 - [Programming Ruby](http://www.amazon.com/Programming-Ruby-1-9-2-0-Programmers/dp/1937785491/) - An older [free addition](http://ruby-doc.com/docs/ProgrammingRuby/) is available online.
-
-
+- [Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide) - A community-driven Ruby coding style guide.