Building with Android Studio

I've added a gradle configuration to the project. This allows importing it to [Android Studio](http://developer.android.com/sdk/installing/studio.html). Gradle will automatically fetch all dependencies, there are no further steps needed, just import and start coding :-)

- all required libraries (except for SherlockNavigationDrawer and android-numberpicker) are fetched from the [maven central repository](http://search.maven.org/) 
- SherlockNavigationDrawer is built from third-party as this is not available as maven dependency
- android-numberpicker is fetched from the [toxbee maven repo](https://github.com/toxbee/mvn-repo/tree/master) because the version found in the maven central repository is not compatible to Android Studio and the library can't be compiled from third-party with gradle without changes to it (which is not a good idea since it is a git submodule)

Tested with Android Studio 0.4.6 on Windows 7 and Ubuntu 13.10