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...)
Yeah.. This is a tricky one.
I changed it to rely on the local android sdk, but it does require you to install the Android Support Repository through the SDK manager. Added a little to the README.