/*
 * Extract the actual artist/album/track ID from a MBE_GetxxxxxId query.
 *
 * The MBE_GETxxxxxId queries return a URL where additional RDF metadata
 * for a given ID can be retrieved.  Callers may wish to extract only
 * the ID of an artist/album/track for reference.
 *
 * Aliases:
 *   MusicBrainz::Client#get_id_from_url
 *
 * Examples:
 *   # get the artist name of the first track on the album
 *   url = mb.result MusicBrainz::Query::AlbumGetArtistId, 1
 *   id = mb.id_from_url url
 *   
 */
static VALUE mb_client_id_from_url(VALUE self, VALUE url) {