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)

Because this code is older than parboiled2 and back then a hand-written version was at least one order of magnitude faster than parboiled. Now the perf benefit of the hand-written parser is much smaller (but still present), but since the code is already written we didn't care to rewrite.