Add support for PHPBB "smiley packs"

Smiley packs are a means of distributing a collection of smileys for PHPBB. It would be neat if Smileys module could parse these, as it makes importing smileys a lot less tedious. (I ended up wussing out and going the route of exporting from PHPBB's smiley table and doing some fancy find/replace on the SQL to do an import back to Drupal, but this would've been a nice way to do it.)