Gradle support

Primary, i adopted the new directory structure from http://tools.android.com/tech-docs/new-build-system/user-guide

Secondary, i deleted your **release** folder, i think releases should be handled through the *new* release function in GitHub.

The project builds fine in the current version of Android Studio and InteliJ.


PS: As a result of having a build system, you should consider filtering the ```FlattrConstants```…