Support Actors that implement IObserver & IObservable

[sdether]

Actors should be able to implement IObservable and IObserver respectively and the ActorContext would set up the proxy to between the two. I.e the subscriber would subscribe like this:

var o = Context.Find<IOberservable<int>>(observableRef);
o.Subscribe(Context.AsObserver(Self));

and the actor behind observableRef would receive the IObserver proxy and calling its methods would in turn generate messages back to the caller's mailbox and delivered to the IObserver methods.