printenv.js: New example to show system.env.

This is very similar to Unix common 'printenv'.

http://code.google.com/p/phantomjs/issues/detail?id=271

examples/printenv.js

@@ -0,0 +1,10 @@
+var system = require('system'),
+    env = system.env,
+    key;
+
+for (key in env) {
+    if (env.hasOwnProperty(key)) {
+        console.log(key + '=' + env[key]);
+    }
+}
+phantom.exit();