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.
the purpose of the rejection is to cut out URLs ending in jpg / gif / png that aren't actually pictures. photobucket, for example, has URLs like this one that will redirect to an actual page, so it's not a direct image link:

http://s157.photobucket.com/user/loki993/media/boardssmall.jpg

I don't think you should need 500px in here unless people ever link to 500px.com/somedir/somefile.jpg and it's not actually a direct image link... I couldn't find examples of people linking that way in a quick search but you may be more familiar than me!