akka-http-core baseline

**preview --- do not merge yet!**

This is an 90% intermediate state of the move of *spray-http* into akka as the new *akka-http-core* module.
It is feature complete with regard to the HTTP model, the main thing that is not yet included is the new HTTP header value and URI parser. We are currently in the process of improving the parsing logic in three ways during the move:

1. Update to the latest [HTTPbis Draft](http://tools.ietf.org/html/draft-ietf-httpbis-p1-messaging-25)
2. Upgrade to parboiled2
3. Merge the header value parser and the URI parser (which are separate in *spray-can* due to performance reasons).

There are three package name spaces:

- `akka.http.model.*` contains all the types that have a "stand-alone" meaning in HTTP
- `akka.http.headers.*` defines the modeled HTTP headers and all supporting types
- `akka.http.parser.*` will contain all the parsing code

The first two packages are mostly done and as such are ready for review.
The `akka.http.parser` package is still mostly empty and will receive the parsing logic as soon as its ready.

We'll commit into this PR to work in your feedback and contribute the missing pieces.