returning a Future from SprayActor.start

this change allows callers to wait for completion before moving on, to make sure their server has started.

this PR also changes the generic name `config` to a more specific `sprayConfig`
@arschles If we had a way to create the ```sprayActor``` using the ```CallingThreadDispatcher``` then you could avoid this await. Currently, this doesn't seem possible because ```SprayActor``` is an object.