Add a contrib/ folder with startup script for Mac OS/X

I wrote a startup script for Mac OS/X that I use to launch ungit immediately after login. I thought it might be useful for other users as well. 

One downside is that it requires that there be symbolic links for ungit and node in /usr/bin, which I document in the README.md file in the contrib/ subdirectory. Unfortunately, I don't know how to add this linking behavior to the installation routine, so I left it out. I assume it can be automatically done, though, on Mac OS/X, so if you would like, I can spend some time looking into this so it automatically installs.