Move sponsor data into _data/.

This also eliminates another hack on the front page that was used to
retrieve the sponsor data for the sponsor slider.
Fix nits.
Remove another unsightly newline.
Remove subdirectories from _data/.

Subdirectories in _data/ are a Jekyll 2.1.0 feature, which GitHub Pages
doesn't support yet.
Formalize the build environment.

Use the github-pages gem to match the local environment with that of
GitHub Pages. Add documentation on how to set up the local environment,
and add a Makefile to simplify serving the site locally while also
enforcing the build environment.
Merge pull request #73 from kevinwang/nits

Fix nits.
Merge pull request #74 from kevinwang/gh-pages-environment

Formalize the build environment.
Merge branch 'master' of github.com:prog694/prog694.github.com