Support event emitting

[jodal]

In Mopidy, we regularly need to emit events from an actor to one or more actors in the stack "layer" above the emitter. I feel that making it easy to listen for and emit events could be a useful part of Pykka.

For reference, Akka implements an EventBus mixin: http://doc.akka.io/docs/akka/2.1.0/scala/event-bus.html