Initial commit of dispatcher key refactoring, for review. See #1458

* Changed signatures and constructor of MessageDispatcherConfigurator
* Changed Dispatchers.lookup, keep configurators instead of dispatchers
* Removed most of the Dispatchers.newX methods, newDispatcher is still there because of priority mailbox
* How should we make it easy to configure priority mailbox?
* Changed tons tests
* Documentation and ScalaDoc is not updated yet
* Some tests in ActorModelSpec are temporary ignored due to failure

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

@@ -104,7 +104,7 @@ class TestKit(_system: ActorSystem) {
   lazy val testActor: ActorRef = {
     val impl = system.asInstanceOf[ActorSystemImpl] //FIXME should we rely on this cast to work here?
     impl.systemActorOf(Props(new TestActor(queue))
-      .copy(dispatcher = new CallingThreadDispatcher(system.dispatcherFactory.prerequisites)),
+      .withDispatcher(CallingThreadDispatcher.ConfigKey),
       "testActor" + TestKit.testActorId.incrementAndGet)
   }