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 
Yes, a `ClassRule` sounds good. Maybe we cold look at this other project for inspiration ;)

https://github.com/akka/akka/blob/cb05725c1ec8a09e9bfd57dd093911dd41c7b288/akka-testkit/src/test/java/akka/testkit/AkkaJUnitActorSystemResource.java