Add basic renderer for 500px images.

This adds a link in the image credits back to the original photo on 500px when unattributed images are linked.
People link directly to images on the 500px CDN but we still want to pick up these images in the 500px module. This allows us to set the credits property to point back to the original image's page on 500px. Without having 500px in the reject list these links are caught by the default module and so we don't get a chance to set credits.

This can be seen in Flickr's case with the top two results here: http://www.reddit.com/search?q=site%3Aflickr.com&sort=relevance&t=all

The first is a link that doesn't get caught by the default module, causing attribution to appear in RES:
http://www.flickr.com/photos/thekog/6886709962/sizes/l/in/set-72157629710255725/

The second does get caught by the default renderer and so the Flickr module doesn't have a chance to add attribution:
http://farm3.static.flickr.com/2648/4018097501_f89ae5e8ca_b.jpg