=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 
What do you mean by "locked" in?
Since the first part of the route (before the `~`) rejected the request it tries to flow into the second part where the `get` also rejects (with a `MethodRejection`). So we need a way for the `put` directive that has matched to cancel out the `MethodRejection` produced by the later `get`.
Note that there is nothing that mandates that those two directives (`put` and `get`) are at the same level. There might other intermediate levels of directives all over the place.