Adds in missing class

Brings in class that was missing from updated Sass but in style.css.
Props @corvannoorloos
Put vendor specific property first
Update _mixins-master.scss
Merge pull request #568 from fsylum/master

Reordering vendor-prefixes property in the mixin: props @fsylum
Puts width in right place

Props @corvannoorloos
indentation using tabs instead of mixed tabs/spaces

see https://github.com/Automattic/_s/pull/565
Merge pull request #570 from corvannoorloos/patch-1

Indentation using tabs instead of mixed tabs/spaces- props @corvannoorloos