Fix relative pathing for when included in training.gihub.com/kit

* Commit that broke this: https://github.com/github/training-kit/commit/c49fc1545fbdfa2e6d3b0c1b28866df04f88a6e3
* Was using {{ site.url }} but that doesn't work when deployed t
* http://training.github.com/kit
* Compromise approach with one test and one var of . or .., used throughout