Introduce parental supervision, BUT TESTS ARE STILL FAILING

- every actor is now supervised, where the root of the tree is
  app.guardian, which has its supervisor field set to a special ActorRef
  obtained from provider.theOneWhoWalksTheBubblesOfSpaceTime (this name
  is meant to indicate that this ref is outside of the universe, cf.
  Michio Kaku)
- changed all tests to obtain specially supervised children (i.e. not
  top-level) via (supervisor ? Props).as[ActorRef].get
- add private[akka] ScalaActorRef.sendSystemMessage for sending
  Supervise()
- everything routing or remote is broken wrt. supervision, as that was
  not “properly” implemented to begin with, will be tackled after
  app/supervision/eventbus/AkkaSpec are stabilized enough

akka-testkit/src/main/scala/akka/testkit/TestKit.scala

@@ -99,7 +99,7 @@ class TestKit(_app: AkkaApplication) {
    * ActorRef of the test actor. Access is provided to enable e.g.
    * registration as message target.
    */
-  val testActor: ActorRef = new LocalActorRef(app, Props(new TestActor(queue)).copy(dispatcher = new CallingThreadDispatcher(app)), "testActor" + TestKit.testActorId.incrementAndGet(), true)
+  val testActor: ActorRef = new LocalActorRef(app, Props(new TestActor(queue)).copy(dispatcher = new CallingThreadDispatcher(app)), app.guardian, "testActor" + TestKit.testActorId.incrementAndGet(), true)
 
   private var end: Duration = Duration.Inf