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/AbstractDispatcher.scala

@@ -80,7 +80,7 @@ final case class TaskInvocation(eventStream: EventStream, runnable: Runnable, cl
       runnable.run()
     } catch {
       // FIXME catching all and continue isn't good for OOME, ticket #1418
-      case e ⇒ eventStream.publish(Error(e, "TaskInvocation", e.getMessage))
+      case e ⇒ eventStream.publish(Error(e, "TaskInvocation", this.getClass, e.getMessage))
     } finally {
       cleanup()
     }