removed unnecessary files from Git, added a Gradle build script

Hey there.
I did some of the improvements /u/0x800 suggested : 
- I removed unnecessary files from Git. See here for what you souldn't put in Git [http://stackoverflow.com/questions/1273921/what-should-not-be-under-source-control](http://stackoverflow.com/questions/1273921/what-should-not-be-under-source-control)

- I added a Gradle build script. This allows you to use Android Studio easily if you like. And it allows other devs to easily build your app. You can see here for more details [http://www.gradleware.com/resources/tech/android](http://www.gradleware.com/resources/tech/android)