WARNING! NEEDS CAREFUL DEPLOYMENT

WARNING! NEEDS CAREFUL DEPLOYMENT
* Bug 9213: Fixed the plainly broken user_newtalk updating and caching scheme. I tried to keep my changes roughly performance-neutral, but the update on Wikimedia should be watched carefully for performance problems.
* Made UserMailer a class, use the autoloader to load it
* General UserMailer refactoring
* If the user has email-on-newtalk enabled, send them an email for every change, not just the first one before they view the page again.
* Don't add a watchlist entry automatically on change of user talk page