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)

Well.... you probably won't gain much. :)
We use the `compressible` flag for avoiding compression of already compressed content.
So in this case a file with extension `svgz` will be compressed twice, which I am not sure will actually work. However, `image/svg+xml` is the content type to be used for both file extensions, so we'd need additional logic to discriminate the two.

I'll remove the `svgz` extensions for now, seeing that it is quite rarely used anyway.