scss: Added SCSS file
scss: Added threshold and width variables
scss: Embedded .left and .right subclasses
scss: Embedded .leaflet-control subclasses
scss: Embedded media queries
scss: Embedded .close subclass
scss: Extracted widths() mixin
scss: Extracted box-sizing() mixin
scss: Extracted border-radius() mixin
gulp: Added sass task as lint:css dependency
scss: Extracted $transition-duration variable
Merge pull request #20 from Turbo87/scss

Converted CSS to SASS