Add ungit to gui list

Add a link to ungit to the list of gui clients (http://git-scm.com/downloads/guis)

Ungit is a web-based git ui which is aimed at making it easy to understand the underlying model of git; see https://github.com/FredrikNoren/ungit for details and http://www.youtube.com/watch?v=hkBVAi3oKvo&feature=youtu.be for an introductory video