Maven POMs for building libGDX with Maven

This will allow you to deploy to Maven Central using the "standard" process, which is pretty easy once you have jumped through all the hoops to get a Sonatype account.

This only creates artifacts for the things that are currently published to the private libGDX repository. There are a bunch of other things in libGDX that would be nice to get into Maven so that they could be easily added to a GDX project that was generated from the Maven archetype, but I'll start here and add the other stuff later.