[2.3] Adds Resource Group tree icons

BEFORE:
![rg_before](https://cloud.githubusercontent.com/assets/352182/3475430/e1fd8512-02f0-11e4-882f-acebcc770685.jpg)

AFTER:
![rg_after](https://cloud.githubusercontent.com/assets/352182/3475412/82226568-02f0-11e4-8ec1-8fb340521896.jpg)