[breaking] Akka needs an ExecutionContext, not an ActorSystem.

This enables users to use Futures without an ActorSystem.  It requires something like this to be added to all clients:

    protected implicit val executor = system.dispatcher

When we drop Scala 2.9, we can add an executor as an implicit param to `asynchronously`, and this abstract def goes away.

Does this add value now, or should we hold off and Do It The 2.10 Way in the next release?