akka-http-core current state

**NOT FOR MERGE YET**

This PR contains the current state of akka-http-core on top of akka-stream.
It compiled but doesn't run yet as `Flow.concatAll` and `Duct.concatAll` is still stubbed.
I don' think it should be a project dependency to `actor`, since we want dependencies to published artifacts here.
Dependency to `stream` is fine (as far as I can see), and it will bring in the right akka-actor.