Add GitHub Ribbon to our lessons

This is related with swcarpentry/site#448.

Long lines isn't nice but in this case it will make easy to remove
the ribbon when build the print and epub version of our lessons by
searching for the line with `id="github-ribbon"`.

TODO:

- Check if the color of the ribbon is OK.

  More colors are available at
  https://github.com/blog/273-github-ribbons.