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 
Yeah simply the multiple `handleWiths` overloads look pretty good actually (for java). Sadly all these come at big boilerplate costs on our end... Let's try the `handleWith`s first :)