__class__ for all AbstractProxy descendants

This provides access to the current proxy class for classes which inherit from
Celluloid::AbstractProxy. This can be used for methods that need reflection.
Implement #respond_to? on SyncProxy (fixes #303)

Check the instance methods of the proxy class first when responding to
respond_to?

If that fails, ask the actor if it responds to the given method.
[BUG] ActorProxy doesn't respond_to respond_to? correctly