=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 
You can't opt out of the current route execution model. Also most directives are as "simple" as the method directives.
Consider for example the `path` directives or other ones like `cookie` or `headerValue`. There is might make perfect sense to nest several incarnations at some levels within each other.
Also you'd need overhead to manage the "lock-in" you are thinking about as well as try to establish "lock-in domains", which is probably not that easy.