Added data-at2x attribute support

I have modified retina.js to support user-specified `data-at2x` attribute specified in `img` tags as the preferred high-resolution image URL. retina.js would not trigger HEAD requests for images that have `data-at2x` attribute specified and would just use the specified URLs as is.

This addresses issue #59 without forcing users to turn off asset digests. But this does require users to manually specify the at2x paths for each image, which could be done with a helper. I have provided a sample helper method in the README.