Split CellBehavior from Actor to allow for simple Erlang-like actors

Sometimes people like @rtyler enjoy writing hardcore `Thread`ed code themselves. 
They need to realise that using `Celluloid` actors without having to depend on the RPC and default `Task` behaviour can make their lives so much better. 

This change allows for actors to have custom behaviour which may not use all of the features that `Celluloid` provides. 
One example is a [Router which simply forwards method calls based on some strategy](https://gist.github.com/halorgium/7dfdd9badb46c63cb804). 

More changes in progress.