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)

It's the counterpart to HttpCredentials. The server may ask some kind of "security question" (HttpChallenge) which the user is supposed to answer (HttpCrendential). E.g. for the Digest `scheme`, one of parameters is the nonce used to prevent replay attacks. (Though, Digest is not directly supported because of its other security flaws.)