upnp: some fixes

These are some fixes from my media import work which IMO are worth submitting already.
* The first commit fixes sorting by upnp:rating. In the upnp specification "upnp:rating" means a "parental rating" and not a "star rating" so mapping it to SortByRating is false and it should be SortByMPAA.
* Platinum adds an "Unknown" genre if there's no genre available so we should remove that again when we parse genres as a client.
* all the date/datetime values we are currently providing/parsing are pretty wrong. The upnp specification uses the W3C datetime format. Unfortunately our W3C datetime serialization/deserialization is incomplete so I added the necessary code to make it complete. Then I made sure that all date/datetime values are serialized/deserialized in/to W3C format.