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, right, thanks!
However, I'll keep the `defaultHostHeader` parameter around because this is public API generally implementing the RFC logic, which might be useful to users even though we (so far) don't use the `defaultHostHeader` parameter ourselves.

(I'll also update the links into the spec to the new RFCs.)