+act - Reworks implementation of ActorSystem shutdown

  * deprecates awaitTermination, shutdown and isTerminated
  * introduces a terminate-method that returns a Future[Unit]
  * introduces a whenTerminated-method that returns a Future[Unit]
  * simplifies the implementation by removing blocking constructs
Nah, there's a Try that needs to be created anyway to populate the Promise, so I caught 2 birds with one stone.