Added "/usr/bin/git" to the searchLocations.

I use the git version that is shipped with Xcode, which resides in "/usr/bin/git". When trying to open a repository I got this error message:

Could not find a git binary version 1.6.0 or higher.
Please make sure there is a git binary in one of the following locations:

	/opt/local/bin/git
	/sw/bin/git
	/opt/git/bin/git
	/usr/local/bin/git
	/usr/local/git/bin/git
	/Users/bler/bin/git

I did not feel like moving my git binary, so I added my location to the list. Considering that my location is the default location for git on OS X I really think it should be in this list. I was surprised that it wasn't.
Merge pull request #281 from codiophile/master

Added "/usr/bin/git" to the searchLocations.