Changed how numbers are parsed because they were parsed in a naïf and
hairy way. Added tests for #2 and #3 (in old Parsec project these are
number 35 and 39 respectively).
* Since Haskell report doesn't say anything about sign, I've made
‘integer’ and ‘float’ parse numbers without sign.
* Removed ‘natural’ parser, it's equal to new ‘integer’ now.
* Renamed ‘naturalOrFloat’ → ‘number’ — this doesn't parse sign too.
* Added new combinator ‘signed’ to parse all sorts of signed numbers.
* For the sake of convenience I've added ‘integer'’, ‘float'’, and
‘number'’ combinators that also can parse signed numbers out of box.
