Making the actor system configurable

The goal is to make the default _ActorSystem_ name configurable.
This is a nice feature if you put your play application inside an akka
cluster.
I'm not sure about putting it under `akka`, since this is used by Akka itself for its own configuration - if Akka were to introduce a new configuration item in future called `actorsystem` or whatever, this would cause problems.

Perhaps `play.plugins.akka.system-name`.