Http: Java Routing API Preview

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 
We cannot do that here without lambdas. That's why there's the alternative syntax seen in `put(putPetHandler)` where you can supply a route yourself and implement like you want in a completely type-safe way. I didn't try yet but hopefully in Java 8 those can be written as lambdas.