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)

This method is used for creating an `ErrorInfo` from an exception message. The parser layer (parboiled) doesn't support the summary/detail split of error information itself but it does create exception messages which always provide a summary before a colon following the detail.
We make use of this message structure here.