libGDX support

This is the start of a new contrib project that lays the foundation for RxJava support for **[libGDX](http://libgdx.badlogicgames.com/)**.

This is still missing tests because it's very difficult to unit test this properly due to the nature of libGDX. However, I'll be using this a lot in my own Android projects.

I'm not sure whether this should be a contrib subproject of RxJava or maybe a separate repo. If you think this should be separate, just decline the PR.