Add desktop override mixin.
Merge branch 'master' into desktop-override-mixin
Completely re-write the desktop override mixin.
Rename desktop-override to style-override and add tests.
Merge branch master from origin.
Make test comments for style-override consistent.
Make head selector configurable as param and var, rewrite and expand comments to better explain how the hack works.
Tweak comments.
Add a function to check if parent-exists and use that to build the selector
Fix test typo
Merge pull request #31 from mobify/desktop-override-mixin

Add a desktop override mixin