ticket 335. implement more media: tags

* media:group (ignored, to get to the tags within)
* media:description (same as summary or description)
* media:credit (goes into media_credit)
* media:restriction (goes into media_restriction)
also support media:license
Support additional media-rss elements

Fixes issue 334.
Fixes issue 335.

Thanks to Paul Swartz for providing these patches!