Cleanup of methods in Actor and ActorContext trait. See #1377

* Added JavaActorContext, UntypedActor.getContext
* implicit val context in Actor needs to be implicit to support forward,
it would be nice if it wasn't implicit because now I can't override context
in UntypedActor
* Removed implicit def system in Actor
* Removed implicit def defaultTimeout in Actor
* Removed receiveTimeout, children, dispatcher, become, unbecome, watch,
unwatch in Actor
* Removed corresponding as above from UntypedActor
* Removed implicit from dispatcher in ActorSystem
* Removed implicit def timeout in TypedActor
* Changed receiveTimeout to use Duration (in api)
* Changed many tests and samples to match new api

akka-docs/scala/code/ActorDocSpec.scala

@@ -13,7 +13,7 @@ import akka.event.Logging
 
 //#my-actor
 class MyActor extends Actor {
-  val log = Logging(system, this)
+  val log = Logging(context.system, this)
   def receive = {
     case "test" ⇒ log.info("received test")
     case _      ⇒ log.info("received unknown message")