Adds a button called 'image source' to the sidebar to download images viewed on 500px.com.
Note: If you attempt to view the image source before the image has finished loading you may end up with a thumbnail.
12/4/2015 - Fixed for new layout, better integration.
8/22/2014 - No longer need to wait for image to finish loading before download button appears.
I'll add support for gallery image modal windows soon. In the mean time opening images in a new tab (middle click) will allow the script to work.