!act,doc #3623++ Removed isTerminated checks from ActorClassification

Instead of isTerminated we now use death watch on subscribers.

! Breaking change - ActorClassification based event buses now require
  and actor system. Previously no actors were involved, but now someone
  has to `watch` the subscribers. If this change is not acceptable, we
  can move to an `init` method based interfacem which is less safe but
  wouldn't break API of `ActorEventBus`
* replaced isTerminated checks with watching actors
* `ActorClassification` starts the unsubscriber instance by itself,
  the unsubscriber is an system actor, and can be stopped via
  `ActorClassification#shutdown`
* Will unregister from unsubscriber, when no more subscriptions for
  given subscriber are left in this bus.
* added missing "Java API: " for some types
* Updated docs to point out the automatic subscriber purging (on terminated)