Improve Flickr translator (Rights (esp. Creative Commons), Medium, Size)

[karnesky]

Primarily: complete the "rights" field using licensing information, per:
​http://forums.zotero.org/discussion/10968/flickr-and-creative-commons/

Perhaps "Medium" should also be auto-filled to denote these are digital and the size could be filled with width/height in pixels, provided by the API.