Add support for become/unbecome

[sdether]

Since we already have a type separation of actors from the messaging contract we can easily swap out what contract an actor supports and with that implement become/unbecome (http://doc.akka.io/docs/akka/snapshot/scala/actors.html)

The trickier part is communicating this to the caller. This will likely end up being a social contract rather than a syntax thing. I.e. the caller would know that after calling the actor method X, the actor will transition to a new contract, so the caller needs to Find a new proxy for the same ActorRef but the new type.