Makes site protocol-neutral (possible to run over HTTP or HTTPS)

If you visit the site right now over HTTPS, at [https://project-open-data.github.io](https://project-open-data.github.io/), you'll see it renders incorrectly, as some stylesheets are not loaded, and the Github "Fork" button at the bottom is missing.

This PR fixes those, so that the site will run properly over either HTTP or HTTPS. This is done by:

* Moving the Github button file into the repo itself, as [github-buttons recommends](https://github.com/mdo/github-buttons#limitations).
* Making the `root_url` of the site [protocol-relative](http://www.paulirish.com/2010/the-protocol-relative-url/), a common practice to make URLs resolve to whatever protocol the site they're contained in is currently using.