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 Any chance it could be ```Future[Event]```? https://github.com/akka/akka/blob/release-2.3/akka-actor/src/main/scala/akka/io/Tcp.scala#L429