Remove jekyll resources from master
Updated readme
Add `background` mixin

Allows adding multiple gradients and background images using shorthand
notation.
Add check for shorthand list containing gradient
Import background shorthand mixin
Update master
Version bump to 2.1.1
Adds image-rendering mixin
Adds image-rendering mixin
Merge pull request #109 from thoughtbot/gf-image-rendering

image-rendering mixin
Avoid deprecation warnings in button add-on (fixes #119)
Update LICENSE copyright
Update Copyright years
Added comments for using prefixer
Merge remote-tracking branch 'linrock/patch-1'
Use Thor for command parsing
Update authors
Add hidpi() mixin
Simplify Opera's ratio
Add hidpi() mixin to the README