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 
A related problem arises with having all those `handleWith` overloads for all the different numbers of arguments: if you get the call wrong you get pages of compiler errors even with just these four defined. Do you have an idea how we could solve this otherwise (i.e. without overloads)?