Fix gradle build

Note that you have to make sure to checkout the submodules as well. Fixes #56.

Also, it seems like a lot of work maintaining three build systems, maybe just pick one? (not sure gradle really is the best one...)
I think we can just rely on the user to have the support repo in their android sdk: http://stackoverflow.com/questions/19958213/how-to-add-android-support-repository-to-android-studio. I'm definitely less sure about this since it requires some arcane knowledge to get it to work while a jar is pretty straightforward.