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
* dded .sass-cache dir to .gitignore