add JavaTestKit, see #1952

- it’s a completely new-written thing in pure Java, so that “protected”
  modifiers actually work and no ghost errors appear wrt. inheriting
  from PartialFunction or similar
- it also features integration with the EventFilter
- all closure-based constructs are modeled as inner classes of the
  JavaTestKit, where the user needs to override a single method which
  will then be executed

akka-testkit/src/test/scala/akka/testkit/TestProbeSpec.scala

@@ -44,10 +44,10 @@ class TestProbeSpec extends AkkaSpec with DefaultTimeout {
       //#autopilot
       val probe = TestProbe()
       probe.setAutoPilot(new TestActor.AutoPilot {
-        def run(sender: ActorRef, msg: Any): Option[TestActor.AutoPilot] =
+        def run(sender: ActorRef, msg: Any): TestActor.AutoPilot =
           msg match {
-            case "stop" ⇒ None
-            case x      ⇒ testActor.tell(x, sender); Some(this)
+            case "stop" ⇒ TestActor.NoAutoPilot
+            case x      ⇒ testActor.tell(x, sender); TestActor.KeepRunning
           }
       })
       //#autopilot