added retina-image mixin
Added optional retina-filename
Added asset-pipeline support for rails
Replaced media-query with hidpi mixin
Removed image-height and width in favor of background-size variable
dont impose 2x naming for custom retina-filenames
Change false values to null to deprecate compact function
Remove deprecated webkit-gradient code
Removed unneeded compact function
Merge branch 'deprecate-webkit-gradient'
Fix merge conflicts. Added custom retina suffix.
Added strip-units function. Make the px-to-em function accept unitless or values with units.
Border-image fix for firefox