Add BasicActor for simple Erlang-like Actors

I still need to figure out a few things, but this creates a more logical separation between the common Actor model as in Erlang and Akka, and the method-invoking and object-based model presented in Celluloid. 

@tarcieri mentioned that @mental and @josevalim might have some input. 

I also need to consider how to configure the actors without the presence of a proxy object. 

Another discussion point: is this better as a composition rather than an inheritance?