Added the 'method' keyword which define methods inside classes.

Example:
	class Fib {
	    method nth(n) {
	        n >= 2 ? (__METHOD__(self, n-2) + __METHOD__(self, n-1)) : n;
	    }
	}

	var fib = Fib();
	say fib.nth(8);   # prints: 21

* Note: the 'feature' of using functions as methods inside classes has been removed.

Added the special '+=' method (as an alias for 'define_method') to declare a method inside a class.
Simplified the parser and improved the performance a little bit.