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)

Roland's suggestion was to simply allow for adding a custom context object to the requests and responses. This way the stream-based API can break the ordering guarantee (i.e. responses can be produced in any order) because the application can rely on its own context object for association of requests and responses.
In fact, I could imagine that in many cases this will be even more convenient for the users because it can remove the need for adding queues on the user's side.