Accept SVG files as well as PNG

Since openbadges-bakery accepts SVG, I've implemented rudimentary support for SVG here too.

I would have liked to have implemented SVG support better in lib/award.js, but it seemed tricky because openbadges-bakery doesn't expose it's SVG-checking abilities, and I wasn't keen on reimplementing the behaviour, so I've went for a safe option for now that checks the URL's extension, and defaults to PNG.