Added an open-project-root command

I have added a way to open a terminal in the project root directory (instead of the folder of the current file). The command is called "atom-terminal:open-project-root" and mapped to to "alt-shift-t" by default. 

I have tested on Mac, Linux and Windows and it works fine. 
Note: This pull request also includes a fix for a windows bug I found. 

The original idea for this came from @diagramatics during #2.