Adding "retina" version of icon.
Updating original icon based on "retina" source. Updating CSS to make use of "retina" icon and properly display new original size icon.
Updating README to include CSS for "retina" version of icon.
Moving background-size property to "retina" CSS declaration and tweaking documentation to match.
Merge pull request #28 from vagari/retina-icon

Add retina icon