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)

Yes and no. The request/response ordering problem is the main one but of course there are probably cases where a message-based API should just fits your actors-based application better. I'd say that we shouldn't cut off support for the "old way" of doings things too early. Deprecation will come soon enough... :)