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 don't think so, both are user-level utility methods independent from the actual usage inside the implementation. Actually you may want to use `dataBytes` when you get an entity from the implementation while you will use `Chunked.apply` if you construct an entity to pass to the implementation. So, they are used in a complementary way.