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/Dispatcher.scala

@@ -59,7 +59,7 @@ class Dispatcher(
           executorService.get() execute invocation
         } catch {
           case e2: RejectedExecutionException ⇒
-            prerequisites.eventStream.publish(Warning("Dispatcher", e2.toString))
+            prerequisites.eventStream.publish(Warning("Dispatcher", this.getClass, e2.toString))
             throw e2
         }
     }