Add context menu option

Added an "Open Terminal at Root" option in the tree view context menu

![2014-09-21 00_47_34-](https://cloud.githubusercontent.com/assets/136959/4347450/d6a22704-4152-11e4-865c-ad06ccaba836.png)

Sorry, I don't have OSX to test if the `darwin.cson` addition works.