Add support for proxying parent::method

[eddawley]

It would be nice if Classy would support setting proxies on calls to parent methods from a child.

For example:

class Ancestor {
  public function foo() {
  }
}

class Child extends Ancestor {
    public function bar() {
       parent::foo();
    }
}

This should be possible by checking both static::__classy_hasProxy and self::__classy_hasProxy.

Also, we need to add some unit tests for this stuff.