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 
But if you're looking at the logs, you won't see that it's due to NotEnoughDataException.

What you could do is to throw the ISE with the NEDE as the cause.