Remove old Actor

Now that `Actor` has been replaced by `Actress` there are a couple of classes that are no longer being used. Specifically, `Actor`, `Postable`, `Runnable`, and `Stoppable`. These classes are all removed completely removed in this PR. Additionally, `Supervisor` has been given an *[EXPERIMENTAL]* warning.

`Supervisor` and `Runnable` were originally designed together to provide behavior similar to Erlang's [supervisor](http://www.erlang.org/doc/man/supervisor.html). Several other classes, such as `Actor` and `TimerTask` included `Runnable` and worked with `Supervisor`. None of those classes exist anymore, leaving `Supervisor` unused.

I've left `Supervisor` in the codebase because I would like to implement classes similar to Erlang's [gen_server](http://www.erlang.org/doc/man/gen_server.html), [gen_event](http://www.erlang.org/doc/man/gen_event.html), and [gen_fsm](http://www.erlang.org/doc/man/gen_fsm.html). At that time I will update `Supervisor` to work with those classes.