Fix broken device pixel ratio query for Firefox

#20 is awesome but the correct syntax for device pixel ratio query in FF is `min--moz-device-pixel-ratio` instead of `-moz-min-device-pixel-ratio`. See [MDN](https://developer.mozilla.org/en/docs/CSS/Media_queries#-moz-device-pixel-ratio).

Also adds `min-resolution` query, which is stated as the standard way in [CSSWG](http://www.w3.org/blog/CSS/2012/06/14/unprefix-webkit-device-pixel-ratio/).