=htp groundwork for routing and a first, basic set of directives

This is a first cut extracted from #15609 ready to be reviewed.

It contains 
 - The basic data structures for routing
 - `BasicDirectives` containing helper directives used for building other directives
 - `PathDirectives` + `MethodDirectives`, so that one can build simple demos and as a demonstration of how directives are built
 - A first implementation of `HttpService` on top of streams
 - a new `akka-http-testkit` project containing our testing infrastructure
 - a new `akka-http-tests` project containing tests for the current state + a simple, standalone test application demonstrating routing on top of streams
 - supporting changes in the existing infrastructure

Even in this form it is quite big and much stuff to review so I'm not sure if it is still viable to be included in the 0.6 milestone.

/cc @sirthias 
This is a point I wanted to discuss.
We did have a purely future based routing DSL at one (early) point in spray, however performance was not great due to all the scheduling. However, purely strict doesn't work either.
Additionally this problem shows up at several places throughout akka-http, e.g. marshalling (mostly strict, but some important exceptions), unmarshalling (same), route completion (same), etc.
So pulling all this out into a common abstraction appears to be the best choice.
What do we do?