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.
Yes. right. Will fix.
(You can't see this change in the other PR because `AkkaBuild.scala` is beyond github's max diff threshold (but it is the only file not being shown there).