Use SASS

Now that [GitHub pages supports a version of Jekyll with
built-in SASS
 support](https://github.com/blog/1867-github-pages-now-runs-jekyll-2-2-0),
it makes sense to use SASS for our stylesheets.

* Update Gemfile to reflect [GitHub pages recommendation](https://help.github.com/articles/using-jekyll-with-pages#installing-jekyll)
* Update README to reflect using bundler
* Update _config.yml to get rid of deprecated settings
* Use SCSS' 'import' instead of Liquid imports to make stylesheets
  modular.  This requires renaming partials with .scss extensions,
  prefixing their name with an underscore, and moving them to the
  _sass directory.
* Define human readable and semantic color names and use throughout
* Nest hierarchical styles where this clears up code
* Use '//' style comments to make output CSS smaller

Fixes https://github.com/openelections/openelections.github.io/issues/12