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/actor/JavaAPI.java

@@ -9,7 +9,7 @@ import static org.junit.Assert.*;
 
 public class JavaAPI {
 
-  private ActorSystem app = new ActorSystem();
+  private ActorSystem app = ActorSystem.create();
 
   @Test void mustBeAbleToCreateActorRefFromClass() {
       ActorRef ref = app.actorOf(JavaAPITestActor.class);