Added ability to specify which dimension should be resized in the retina media query.  Added more browser prefixes.

I found some cases where I wanted the height to be set and the width at auto (for example, a horizontally spanning background image) so I've added a new variable to the @mixins.  Width is still the default, but if you add a second variable called "height" it will set that value instead.

Also, expanded the media queries to include a bunch more browser prefixes.