=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 
No, I don't think that this would compile. The result of `&` would be a `StandardRoute` in this case which doesn't expect an inner route.

FWIW my previous suggestion was to restrict this implicit to `Directive0` and instead just allow this syntax:

```scala
path(Segment).ignoreExtractions & complete("I don’t care")
```

which would be good enough for the rare cases where actually ignoring the extractions is desired.