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)

Yeah, true. (Even though I just adopted the your funky name for this beast).
If you have a proposal for how me might improve the rendering operators please go for it.
We are not at all married to them as they are.