Prep for Jekyll 2.0.0

Get stuff ready for Jekyll 2.0.0

- [x] Convert to Sass
- [x] Break out styling into partials
- [x] Use variables for colors (including `darken` and `lighten`)
- [x] Make some cosmetic upgrades for the funsies
- [x] Upgrade to latest Pure version
- [x] Upgrade to latest Font Awesome version
- [x] Deal with Sass files being part of `site.pages` collection
- [x] Bump `github-pages` gem