upnp: several additions and improvements

These are most of the additions to UPnP and the Platinum SDK from my media import work.
* proper support for dc:publisher in Platinum SDK. This is filled with the studio for video items and can later be used to e.g. fill in the label that produced a music album.
* support for upnp:episodeCount and upnp:episodeSeason from ContentDirectory:4 specification in Platinum
* support for xbmc namespace and the properties dateadded, rating, votes and artwork to get additional details to the client
* implementation of the Search() method from ContentDirectory:1 specification in Platinum. This allows to e.g. search for all items of a specific type
* implementation of the GetSearchCapabilities() method from ContentDirectory:1 specification in Platinum. This allows to get a list of supported search fields/properties. Right now we only support "upnp:class" i.e. searching by the type of media item
* implementation of the GetSortCapabilities() method from ContentDirectory:1 specification in Platinum. This allows to get a list of properties by which a list of items can be sorted. Furthermore I've implemented sorting for upnp:episodeCount, upnp:episodeSeason, xbmc:rating, xbmc:dateadded and xbmc:votes