season artwork support

after the changes made to JSON-RCP allowing to update season details I made some changes to the script that maybe useful until its possible to have the ListItem.SeasonId in the GUI.

It can be run with the command
XBMC.RunScript(script.artwork.downloader, mode=gui, mediatype=season, dbid=$INFO[ListItem.DBID], seasonno=$INFO[ListItem.Label]

It just needs the right condition, so it can be run from the DialogVideoInfo