SRT Moved to GitHub

Since Git as a VCS has become more prominent and the popularity of the service GitHub has (rightfully) skyrocketed, I finally got around to putting the Simple SDL RayTracer onto GitHub.  Not everyone in the industry uses SVN so it's high time I got comfortable with something else too.  If you've been holding off on Git because the clients have been bad I don't really blame you.  But they have gotten much better since I first tried them out, especially on Windows, and are frequently being offered as plugins for IDEs if that's the sorta thing you're into.