Gradle support

Gasp! That gave me headaches! After a few more hours, I managed to do achieve basic Gradle support: Project can be opened and built in Android Studio. Here's what I've done:
* build.gradle files for main and sub-projects
* Fixed some redundancies in the resources (Build system complained about it)
* Moved to API v19

Except for the last bullet (API update), the traditional builds with Eclipse should not be affected by this.

Anyway, there are a few things to care about in the future:
* Different Android Support libraries: The submodules and the main project use different versions (Android Support, Google Play Services). In Android Studio, external libraries can be referenced like this: http://developer.android.com/google/play-services/setup.html#Setup 
* `.gitignore` files aren't perfect yet. We need some more experience for a better tuning. For the moment, I excluded the whole `.idea` directory.