Make imageset retina capable

With the addition of fontawsome its clear a lot of our assets are low res. We need to fix this if we can. The ultimate fix will be to start supporting and using svg everywhere but this is not doable until 3.3.

With that said there is a way we can upgrade the image quality of our assets without affecting any bc by simply swapping out the current files for double sized png versions and defining a 'background-size' on the current class that renders the images.