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-actor/src/main/scala/akka/dispatch/Mailbox.scala

@@ -214,7 +214,7 @@ private[akka] abstract class Mailbox(val actor: ActorCell) extends MessageQueue
       }
     } catch {
       case e ⇒
-        actor.system.eventStream.publish(Error(e, actor.self.path.toString, "exception during processing system messages, dropping " + SystemMessage.size(nextMessage) + " messages!"))
+        actor.system.eventStream.publish(Error(e, actor.self.path.toString, actor.actor.getClass, "exception during processing system messages, dropping " + SystemMessage.size(nextMessage) + " messages!"))
         throw e
     }
   }