Add TNs for WorldCat, another way to append links back to the object.

To see TNs in action: http://www.worldcat.org/search?q=on%3ADGCNT+https%3A%2F%2Fdigital.library.yorku.ca%2Foai2+YOU&qt=results_page (@ruebot++)
I can do a pull request against your pull request if you. Thinking we should add a bit about metadata mapping in Digital Collections Gateway.