Work on `gitx`

This is based on #301.

Modifications :
- Adds more AppleScript so we can ask the main application to perform actions on our behalf instead of calling `git`. This effectively removes the dependency on `git` ;-).
	Note that I *added* a dependency on Objective-Git, so that `gitx` can warn execution outside of a working copy without distracting you by making GitX do it.
- Make the `gitx` argument parsing handle file-system paths.
	This allows one to call `gitx path/to/submodule`, or `gitx ..` and have the window open at the correct location (see #8352816 for caveats though).
- Adds an integrated Info.plist file to `gitx` so it doesn't rely on GitX for `--version`.
- Various bells and whistles.