akka-http-core for merge

This is the latest state of akka-http-core:

- Everything wired up properly against current akka-stream
- All tests green
- `test:run` gives you an HTTP server on port 8080 to play with

After merge these things are still missing for M1:

- Merge of Java API (basically ready but not yet part of this PR)
- client-side request rendering / response parsing (in progress)
- docs (in progress)

Right now all methods in this package object have been marked `private[http]`, so we are free to move things into a normal object whenever we like.
Also, since nothing is public API anymore there is no need for any Java counterparts.