Attempted to fix CSS and JS links to be relative.

I intend this as a partial fix for Project-Interoperability/project-interoperability.github.io#17

The link in `footer.html` to http://ghbtns.com/github-btn.html is more problematic, since it appears that site doesn't support HTTPS.

I don't know how to build and try the site locally, so this change would need to be tested somehow before merging it.