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.
Play's Configuration already has a getString helper that returns Option. So can also be something like:
```scala
val name = app.configuration.getString("play.plugins.akka.actor-system").getOrElse("application")
val system = ActorSystem(name, app.configuration.underlying, app.classloader)
```