change source tag in log events from AnyRef to String

- ensure that no “complex” things are attached to a LogEvent (think
  serialization)
- ensure no escaping the “this” reference via LoggingBus during
  constructors (e.g. ActorSystem)
- change it so that
    + Actor/ActorRef are represented by their address
    + Class[_] by simpleName
    + String by itself
- this means that people need to think a little more while deciding how
  “this” should look like in logging (which I think is a good thing)

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

@@ -99,7 +99,7 @@ class Dispatcher(
           executorService.get() execute invocation
         } catch {
           case e2: RejectedExecutionException ⇒
-            prerequisites.eventStream.publish(Warning(this, e2.toString))
+            prerequisites.eventStream.publish(Warning("Dispatcher", e2.toString))
             throw e2
         }
     }