Actor support

Re-did `ActorSupport`, so it now shuts down `GatlingActorSystem` after a test. Since `GatlingActorSystem` is a singleton (and various bits of Gatling depend on this fact), this necessitated a global lock (specs2 tries to run tests in parallel, if it can), but this can be removed if `GatlingActorSystem` is de-singleton-ised, without further changes to tests that depend on `ActorSupport`.