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/scala/akka/dispatch/MailboxConfigSpec.scala

@@ -80,7 +80,7 @@ abstract class MailboxSpec extends AkkaSpec with BeforeAndAfterAll with BeforeAn
     result
   }
 
-  def createMessageInvocation(msg: Any): Envelope = Envelope(msg, app.deadLetters)
+  def createMessageInvocation(msg: Any): Envelope = Envelope(msg, system.deadLetters)
 
   def ensureInitialMailboxState(config: MailboxType, q: Mailbox) {
     q must not be null