Update media query for more high-res-ish screens

Using latest recommendations from [Chris Coyier](http://css-tricks.com/snippets/css/retina-display-media-query/) (via Brett Jankord) to make a more inclusive, still simple media query, and extended 2x images to any display at minimum 1.3x normal pixel ratio (includes Nexus 7), because the 2x images should still be an improvement on those devices