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)

`listeners` contains only the bound servers, so usually very few.
`connectors` has one entry per host that the application has been recently talking to. Theoretically this could be thousands, but this type of application is rare. So far the size of the `connectors` map never appears to create any problems here.