Adds ALAssetsGroup support

Hi,

First, thank you for this project. Saves a lot of time!

This pull request adds ALAssetsGroup support (which can be turned on/off by a UISwitch).

Cheers!

![ios simulator screen shot mar 14 2014 1 31 13 am](https://f.cloud.github.com/assets/139272/2418946/351d8a80-ab53-11e3-8b67-24d5e355d3bf.png)