Modified purge behavior.  Fixed some item counts.

List of items to be purged from a feed will now include any items tagged only with 'folded', and will now exclude the latest n items which would otherwise be purged, where n is the larger of either the number of items the feed source most recently provided, or the value of the 'purge_grace' admin preference.

feed-details.php now shows a rudimentary history of item updates.  Fixed some issues with admin view of non-subscribed feeds, as well.

fof_db_feed_counts() now returns proper number of items in a feed.
Show a little graph of feed activity.

The feed-detail.php page now includes a graph detailing when items were added to a feed.