introducing: MainBus feat. LoggingBus

most tests passing, everything compiling, but docs not updated and nasty
thread-leak preventing me from running the whole test-suite (which is
the reason for this commit: I want to chase down that one first).

- the app.mainbus is classified by Class[_] (currently lookup, will
  possibly change to sub-class-aware) and accepts AnyRef messages
- LoggingBus handles akka.event-handlers from config specially:
  + start them as system services, supervised by SystemGuardian
  + keep their subscriptions in sync when logLevel_= is called
  + send them InitializeLogger(bus) message before subscribing them (so
    they can register for extras like Mute/UnMute)
- two-phased start-up: first phase with actor-less stdout logging, then
  subscription of config loggers, then remove stdout logger (logLevels
  configurable separately)
- MainBusReaper watches registered receivers and unsubscribes them upon
  death (started in phase 2)
- logger factory on Logging object, needs app/bus and log source;
  default instance in app.log

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

@@ -99,7 +99,8 @@ 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)), app.guardian, "testActor" + TestKit.testActorId.incrementAndGet(), true)
+  val testActor: ActorRef = app.systemActorOf(Props(new TestActor(queue)).copy(dispatcher = new CallingThreadDispatcher(app)),
+    "testActor" + TestKit.testActorId.incrementAndGet)
 
   private var end: Duration = Duration.Inf