Gradle-ize project, clarify README

Hey Jeremy,

I took a stab at gradle-izing this project, and I'm successfully using it as a git submodule in a gradle built Glass app.

The changes required consist mainly of:
 - reorganizing the directory structure
 - adding `build.gradle` describing how to build the project
 - adding `src/main/AndroidManifest.xml` giving the project a package name