Add support for building using Gradle

Gradle is the [next-gen build system](http://tools.android.com/tech-docs/new-build-system) for Android. It is used in combination with [Android Studio](http://developer.android.com/sdk/installing/studio.html) or as a standalone tool from console.

This change is pretty basic and doesn’t ruin anything, just adds an ability to build the app using Gradle + Android Studio as well as Ant + Eclipse. In perspective, Gradle allows to remove inner dependencies at the `src` and `libs` directories, but this change will require to abandon Eclipse in favour of Android Studio. Does it make sense or current approach is totally fine with everyone?