fix for karma_comment_statistics update & configurable rating ranges

There is a bug where if a user crosses the minimum karma threshold in either direction, the karma_comment_statistics were not updated for all their comments, even thought some of their comments could reappear or disappear in a node for untrusted users. This led to incorrect comment counts. This patch fixes that problem, and adds a configuration allowing the admin to adjust the max rating from either 3, 4 (default), or 5. This works fine for drop-down lists, but the presentation of the stars option need to be changed to work for a max of 3 or 5.