add github contribute link to footer of all qunitjs.com pages

I've added the "Open an Issue or Submit a Pull Request on GitHub" button to the footer of all qunitjs pages. The footer should look like:
![Screen Shot 2013-01-24 at 10 32 06 AM](https://f.cloud.github.com/assets/475621/93974/72cdaeea-663b-11e2-8154-0f59ae92530d.png)