Stopping and Actor should stop its children

[drozzy]

I think this is one of the biggest selling points of an actor system. Stopping an actor SHOULD stop all of its children.

Without this, we are back in the land of "wild threads" and memory leaks.

This behaviour should be a direct consequence of Stopping actors. See #46.

[jodal]

Ref. http://www.pykka.org/en/latest/#what-pykka-is-not, Pykka doesn't currently have a supervisor hierarchy, which is a precursor for stopping children when the parent is stopped.

I'm open to adding this in the future.