Add support for SASS/SCSS

Refactored less files to be more scss friendly

 * Ensured no variables are used before assignment
 * Removed interpolated variable usage with similar substitute

Added SCSS Grunt tasks
 * convert_less - Converts theme less files to scss
 * swatch_scss - Same as original swatch task, but using sass
 * compress_scss - Same as original compress task, but using sass

Added dependencies for SCSS

Updated _config.yml so jekyll includes theme _bootswatch.scss and _variables.scss files

Added .sass-cache dir to .gitignore