Dispatcher monitoring

Do not merge yet, this is just for initial discussion. 

This PR contains code that watches the thread pools for the different dispatchers in an actor system. This is particularly useful if some of the dispatchers wrap around a lot of blocking calls.

It introduces several new aspects:

* ``akka.pool.thread.count`` the current thread count in the pool (FJ, TP)
* ``akka.pool.running.thread.count`` the "busy" threads in the pool (FJ, TP)
* ``akka.pool.queued.task.count`` the total number of queued tasks (FJ, TP)
* ``akka.pool.size`` the pool size (FJ, TP)

The aspects are tagged with the actor paths, types, and two new tags in form of: ``akka.system:$SYSTEM_NAME``, and ``akka.dispatcher:$DISPATCHER_ID``, where ``$SYSTEM_NAME`` is the ``ActorSystem``'s name, and ``$DISPATCHER_ID`` is the Akka ``Dispatcher`` id.

See http://doc.akka.io/docs/akka/snapshot/scala/dispatchers.html for dispatcher configuration and further discussion of the dispatchers.

Note that this is a brand new aspect ``DispatcherMonitoringAspect``, which should be included in the ``META-INF/aop.xml`` configuration file to see it working. See the ``aop.xml`` file in ``example-akka`` for default settings.