Add rss enclosure support for YouTube, Google Video and custom URL - Add MRSS support

Patch develops _data functions for youtube, google, and zzz_custom_url.

Google addition gets the single item RSS feed for each video to gather metadata. This includes a very full MRSS section, this includes the likes of duration but sadly doesn't include enclosures so size has to be gathered otherwise (see zzz_custom_url method).

zzz_custom_url gathers data using a new function that calls the HTTP response headers for the file, which includes the mime-type and the size. This maybe worth extending to the allowed media types section, as it is almost more reliable than the file extension at identifying the mime-type. This detection would also work for images and audio if anyone is to develop enclosures or other meta-data for them.

YouTube, also doesn't provide a single item feed or information - that I could find anyway. So it uses the same method as zzz_custom_url. From experience with implementation it seems YouTube actually just puts a standard size in anyway though.

BlipTV was already collecting _data from MRSS, now additional fields are stored to be used in outgoing MRSS.

To make merging easier, and use more consistent, all data passed, also from emaudio, for RSS is an array rather than an object.

Where there is appropriate data from the providers in addition to an RSS2 enclosure the MRSS namespace is added and media:content and media:thumbnail are included. Each will be produced only if they have enough data to validate.

Emthumb is given the opportunity to override this. In the future I'm guessing an EmMetadata (or an extension on the Remote Enclosures) could be allowed to do similar.