Add `season-all-poster' and `season-all-banner` support. Updated and reordered things so things are a bit more standardized. Tweak some logging/docstrings to reflect correct verbiage.
Note: It appears that the retrieveShowMetadata tries to work off the currently selected provider's metadata, but really in fact it only works off tvshow.nfo (xbmc/xbmc12+) -- this really should be moved out of generic and just into XBMC only (which xbmc12 subclasses from) or rewritten to support other metadata format (which is only mediabrowser at this time).