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.
```scala
val name: Option[String] = try {
  Some(config.getString("play.plugins.akka.actor-system"))
} catch {
  case m: com.typesafe.config.ConfigException.Missing => None
  // Consider exception other than Missing as fatal there
}
val system = ActorSystem(name.getOrElse("application"), config, app.classloader)
```