assets: SVG badge for Retina displays

A blue badge generated with http://shields.io/. Keeping the PNG for existing badges in the wild.
badge: baseline adjustment to SVG

https://github.com/badges/shields/commit/03e7013bf5f8e8bd207f27c826123e46bef31686
Merge pull request #168 from nathany/svg_badge

assets: SVG badge for Retina displays