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)

I’ve been told by @retronym that package objects are an evil hack, so the reason why I would vote against this particular addition is that the functionality should be offered in a normal object. It is a Scala-only expression of a certain feature, and I agree with Patrik that as such it must also have a Java-idiomatic counterpart.