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)

We save 32 bits everywhere where a `qValue` is used.
If qValues were commonly used I'd agree with you and trade in the memory for the added convenience. However, our impression is quality values are rarely used in REST APIs, so the support for them is more from a correctness/completeness of implementation perspective and therefore we went for the memory savings.