Get Badge HTML and certificate JSON from dataset URL

This PR builds on the stuff I did a while ago to get badge code from the dataset URL. We can now get the raw HTML for a badge (useful for server side integration, which I think DGU will want to do), and also the JSON for a certificate. I've changed the URL to be a bit more sensible too (but kept the old one in case anyone's using it - It's not documented, so I doubt they will be). It's now `/certificates/from_url{.format}?datasetUrl={whevs}`, rather than just `get_badge?datasetUrl={whevs}`, which probably isn't the best.

I'm just rendering these at the URI at the mo, but maybe a redirect to the canonical URL would be better - @JeniT, opinions?