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)

The question is: if we want to allow user access to `enhanceByteArray` and `enhanceString` we need to make them accessible somewhere.
Maybe you already have a better place in akka where this stuff should live?
If not then I'm also ok with simply removing it from the public API altogether. Some spray users will wine about this but if it's akka policy to not provide very "general" helpers then we can point to that.