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 
@jrudolph for OS X it depends on the version of the Java 8 JDK, right now on my machine `/Library/Java/JavaVirtualMachines/jdk1.8.0_25.jdk/Contents/Home`. There is no official symlink :scream:, but you can find out where it is by running `/usr/libexec/java_home -v 1.8`.  The first path `/usr/local/share/java/jdk8` is how I have set it up on my machine :wink:.