added retina-image mixin
Added optional retina-filename
Added asset-pipeline support for rails
Prefixer can have prefixes enabled or disabled
Added keyframes mixin that prevents redundant prefixes in its contents
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
Make prefixer and keyframes mixins more concise