- Took all "rating logic" out of the account.module and moved it into
  a seperate module called "rating.module".  This should allow people
  to experiment with different rating heuristics/algorithms.

- The rating module also generates a "Top 100 users" page, see:
     http://drop.org/module.php?mod=rating

- Adjusted ./scripts/php-clean to ignore png files.
  (I should do it the way around and make it ignore everything but
  our php files.)