Implement ES6 Arrow Function

This patch is implementing Arrow Function syntax.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/arrow_functions
http://wiki.ecmascript.org/doku.php?id=harmony:arrow_function_syntax

I've used the current ES6 draft (May 14, 2013 Draft) for this patch.

 * No support Rest parameters.
 * I'm not sure about the specification of sctrict mode.