* Update decorators parsing
This commit introduces three changes:
1) Class properties can be decorated
2) Decorators can contain arbitrary expressions, using @(...)
3) The Decorator node type has a new property, "arguments". This
makes it possible do distinguish @dec() and @(dec()), which have
different behaviors because @(dec()) is equivalent to @(dec())().
* Rename Decorator#expression to Decorator#callee
* Add test for @dec()()
