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 problem is that your proposal would break the DSL.
`~` has highest precedence and the rendering "DSL" depends on pure left associativity.
So, if we add any operator (or method) starting with a character of less-than-highest precedence things like this will not work anymore (without parens):

    this appendHex i.toLong ~~ something