=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 
The ability to treat a route like a directive is only needed for being able to use it in directive composition.
However, this feature turned out to be not very widely used and admittedly does open some potential loop holes.
Therefore I'd be fine with removing the feature entirely, together with the "StandardRoute" abstraction.
Streamlines things and improves clarity. If it turns out that we really need it we can always add it back later.