Added autoprefixer-rails gem
Removed vendor prefixes from SASS

Prefixes can be handled by autoprefixer
Added a lot of SASS variables for colours
Fixed undefined SASS variable
Added a scss-lint config file
Updated scss-lint config to prefer double quotes
SASS Styleguide updates
Syntax changes to the 404 page's SASS
Syntax changes for the contributing page's SASS
Syntax changes to home-page menu's SASS
Syntax changes to site footer's SASS
Syntax changes to site header's SASS
Syntax updates to article header's SASS file
Syntax changes to base SASS
Syntax changes to site's typography stylesheet
Merge remote-tracking branch 'origin/sass-organisation'