- create ActorSystemImpl trait to make ActorSystem fully abstract
- add Java API for constructing (ActorSystem.create(...))
- only go through factory methods because .start() has become necessary
- rename all user-facing occurrences of “app” to “system” (Actor trait
and TestKit/AkkaSpec)
- pass ActorSystemImpl to ActorRefs upon creation, which means that
actorOf() and friends need such an argument, which must be provided to
the ActorRefProvider by the ActorRefFactory implementation
(cascading into all dispatchers, mailboxes, other stuff; had to move
deadLetter stuff to ActorSystem again and split its initialization due
to cyclic dependency)
BalancingDispatcherModelSpec is currently broken, leaves count==1 in
wavesOfActors test, committed anyway in order to check out where it
broke before this merge
