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, it currently relies on the assumption that the test runner assures that tests are sandwiched between the `BeforeClass` / `AfterClass` methods. I cannot say for sure under which conditions this is actually true. 

I'll try to change it to use `ClassRule` as suggested here: http://stackoverflow.com/questions/12485457/junit-beforeclass-and-afterclass-behavior-in-case-of-multiple-test-cases