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)

It needs to be public because it is part of the model. E.g. here is the definition of a few headers relying on `DateTime`:

    case class Date(date: DateTime) extends ModeledHeader
    case class `Last-Modified`(date: DateTime) extends ModeledHeader
    case class `If-Modified-Since`(date: DateTime) extends ModeledHeader