add Class[_] to LogEvent

- it is customary to use class name for categorizing logs, hence we
  should support it; class is taken from logSource.getClass
- update SLF4J module to use logClass as category and set logSource in
  MDC "akkaSource"
- add docs

akka-docs/scala/code/akka/docs/actor/ActorDocSpec.scala

@@ -162,10 +162,10 @@ class ActorDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
     system.eventStream.subscribe(testActor, classOf[Logging.Info])
 
     myActor ! "test"
-    expectMsgPF(1 second) { case Logging.Info(_, "received test") ⇒ true }
+    expectMsgPF(1 second) { case Logging.Info(_, _, "received test") ⇒ true }
 
     myActor ! "unknown"
-    expectMsgPF(1 second) { case Logging.Info(_, "received unknown message") ⇒ true }
+    expectMsgPF(1 second) { case Logging.Info(_, _, "received unknown message") ⇒ true }
 
     system.eventStream.unsubscribe(testActor)
     system.eventStream.publish(TestEvent.UnMute(filter))