!act,doc #2922 Doc event bus and fix Java API

* Add examples of how to implement the different event bus abstractions
* The Java API was not working, so I created a new Java API (reusing the scala implementation).
  Subclassing the abstract classes in akka.event.japi didn't compile for other types of E, S, C than Object.
  Minimized problem description in akka-dev: https://groups.google.com/forum/#!msg/akka-dev/GojW1CZYXOE/fFotK9myddIJ