Added Gitx-dev

GitX-dev includes a selection of improvements from around the GitX fork community.

The awesome branch/remote/tag sidebar from GitX (L)
Clickable commit references in blame view
There are also a range of visible and under-the-hood changes to make GitX-dev a distinct improvement on other forks you may find.

* Notably better performance on large repositories
* Reliable in-app updates thanks to Sparkle
* Significantly reduced (and shrinking!) use of git command-line tool in favour of direct use of libgit2 and ObjectiveGit.framework
* Lower, more regular memory footprint due to porting to Objective-C Automatic Reference Counting
* Improvements to the command-line gitx tool