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 `ix < 0` an `IndexOutOfBoundsException` is thrown, which is fine.