Make template site easier to customize

Used the new template site (which is awesome) for the first time. It should be a lot easier to customize right off the bat, especially for new users who may not be as familiar with Jekyll. A few things I noticed:

* Asset URLs were hard coded to domain root, making it impossible to use on GitHub Enterprise, project pages, etc.
* Text was hard coded in footer, with no clear explanation of how to change it. Moved to variables and made a note that they can be changed in `_config.yml`
* Pages lived in `about/index.html`, rather than simply using a pretty permalinks
* Header nav hardcoded pages
* Use the default markdown processor

Three questions:

1. Worth adding a Gemfile with Jekyll in it so that the site's ready to go?
2. Should we add a readme to the folder so (e.g., like Rails does)?
3. What's the preferred design pattern for prepending asset URLs with the proper path in production, but not locally?

/cc @jglovier (the template's great!)