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 
How far do we want to go for these? Reason I'm asking is that I've seen a surprising amount of people using akka already with j8, and for those people these methods look "pretty good" already (so maybe worth to cope with the boilerplate and generate up to 22 HandlerN's?)