Adds retina image tag support to the asset-pipeline. This fully supports using image digests while still being able to server hidpi versions of images based on browser pixel ratio. The asset:image taglib will check if an image with the same name suffixed with @2x exists in the same path. If it does, it will add this path to an attribute hidpi_src="". This is automatically scanned by retina_tag.js on dom loaded and can be refreshed manually. It also will, in real time detect pixel width changes and toggle between the lowdpi or hidpi versions.