Github Pages support

Repositories with a `gh-pages` branch will have a website hosted by Github with the contents of that branch.  We can use Github Pages to host the example slideshow.  This change adds support for the gh-pages branch by:

- moving the `example.org` file to `index.org`
- changing the submodules to the git protocol

After applying this change, add a gh-pages branch by running `git symbolic-ref refs/heads/gh-pages refs/heads/master` then `git push --all`.  This will make the `gh-pages` branch track master.  To keep the Github Pages site up-to-date, you will need to run `git push --all` after committing to master.

Demo at http://jblomo.github.com/org-html-slideshow/