http: new (un)marshalling infrastructure

This PR provides the first bits of the new akka-http module: the new (un)marshalling infrastructure required by @huntc. It ports everything we have in *spray* into a new an improved streamified (un)marshalling design (incl. multipart support).

/cc @jrudolph 
    if (parts.isEmpty) Empty else MultipartFormData(SynchronousProducerFromIterable[BodyPart](parts.toList))