Add Link to Brackets-Dev on Help Menu

Addresses issue #8106

Adds a link to the Brackets Google Group to the Help Menu.  Seemed like a good idea.  Now I'm going to go read the newsgroup!  :)

![bracketshelpmenu](https://cloud.githubusercontent.com/assets/7686073/3267164/d0a01e1e-f2c5-11e3-9145-6222d92e6aa3.JPG)