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 are certainly not against using something else that has equivalent perf characteristics.
However, since rendering a `Date` header is required for all server responses (and we parse a `Date` header for all responses received) something light, fast (and immutable) is needed, i.e. `java.util.Date` et al are out of the picture.