Add support for Android Studio and empty state for views.

Hi,

This pull request contains 2 changes:

1. Added support for Android Studio:

    1. Added gradle build script (`build.gradle`).

    2. Moved all the files from the `library/` folder to the root of the repository. Otherwise Android Studio detects the module's name as "library" (because the `build.xml` / `build.gradle` is under that folder).

2. Added support for empty state (`ContentState.EMPTY`) in views. This is useful in cases where the content varies depending on user actions, for instance when a search returns no results. Also this library doesn't play well with [`AdapterView#setEmptyView(View)`](http://developer.android.com/reference/android/widget/AdapterView.html#setEmptyView%28android.view.View%29), so that can't be used.

Please let me know if further changes are to be made.