Http: First step of Java Routing API

Here's the first batch of the high-level Java server API which already demonstrates a few concepts. Looking at the examples already explains much but I will write a summary as we go along as well.

Implementation is also already available (https://github.com/spray/akka/tree/w/15911-java-routing-dsl-implementation) for everything demonstrated here but omitted here for clarity.

/cc @sirthias 
I took a middle route: there's now an ExternalResource you can implement (or adapt) to provide an system. Otherwise, the `JUnitRouteTest` class provides an instance of the resource as a `@Rule` that recreates an ActorSystem per test which seems to work well and is as safe as possible.