clean up initialization of ActorSystem, fixes #1050

- create ActorSystemImpl trait to make ActorSystem fully abstract
- add Java API for constructing (ActorSystem.create(...))
- only go through factory methods because .start() has become necessary
- rename all user-facing occurrences of “app” to “system” (Actor trait
  and TestKit/AkkaSpec)
- pass ActorSystemImpl to ActorRefs upon creation, which means that
  actorOf() and friends need such an argument, which must be provided to
  the ActorRefProvider by the ActorRefFactory implementation

akka-actor-tests/src/test/java/akka/dispatch/JavaFutureTests.java

@@ -19,7 +19,7 @@ import scala.Right;
 
 public class JavaFutureTests {
 
-    private final ActorSystem app = new ActorSystem();
+    private final ActorSystem app = ActorSystem.create();
     private final Timeout t = app.AkkaConfig().ActorTimeout();
     private final FutureFactory ff = new FutureFactory(app.dispatcher(), t);