Materials tree collapsable

How it was before:

![screen shot 2014-06-06 at 1 14 31 pm](https://cloud.githubusercontent.com/assets/6341989/3204357/bbb321e0-eda7-11e3-9ee5-04f96ac476b4.png)

How it appears now:
![screen shot 2014-06-06 at 1 14 26 pm](https://cloud.githubusercontent.com/assets/6341989/3204363/d906ed9e-eda7-11e3-8371-330eaaa1898f.png)