Feature next previous change actions

I've added navigate to next and previous git change menu actions.
I also refactored the GitDiff menu, giving it it's own submenu.

I had to add some Xcode private headers for this to work, I hope that's fine.

![screen shot 2014-09-30 at 09 43 17](https://cloud.githubusercontent.com/assets/57446/4454783/78b9dce8-4875-11e4-864c-e140cb7379a1.PNG)