Add icon of folder for folder node in context menu of Project Panel.

Before:
![2014-12-20 17 47 29](https://cloud.githubusercontent.com/assets/1700940/5515171/ba3802fe-8870-11e4-83e4-5c0477b5ed10.png)

After:
![2014-12-20 17 39 09](https://cloud.githubusercontent.com/assets/1700940/5515173/c6658a92-8870-11e4-862e-80f9b99ca12d.png)