Replaced integer checks that used preg_match() to use ctype_digit() instead

Added (very basic) spam filtering plugin support. Plugins can mark a message as spam by hooking into the spam check API, which is documented in functions.php. No spam checking functionality is built-in.

Login: reauth: window.location.hash is now updated to include the new SID so that page reloads will use it

AJAX core library: possible breaking change, readystatechange functions are now called with the XHR instance as the first parameter, to allow requests to run in parallel. This means much better stability but may break some applets (compatibility hack is included)