Improve page performance by storing user karma totals on the user object

-Added index on user_id column of karma_points table to improve look-up times by user
-Added total_karma column to users table to avoid summing up karma points with each page request
-Added index on total_karma column to improve look-up times by total_karma
-Added 'update_user_karma' method to karma_point which is called after_save to update the users total karma column