+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
Previously the terminationPromise was completed only from `stop`, now the failure is propagated earlier. I don't know if it matters.