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

@@ -613,3 +613,7 @@ object TestProbe {
 trait ImplicitSender { this: TestKit ⇒
   implicit def self = testActor
 }
+
+trait DefaultTimeout { this: TestKit ⇒
+  implicit val timeout = system.settings.ActorTimeout
+}