New tidy function that uses html tidy (http://tidy.sf.net) to make sure the output...

New tidy function that uses html tidy (tidy.sf.net) to make sure the output is valid xhtml/xml
Disabled by default, set $wgUseTidy = true to enable
Possible improvements:
* Tweaking it to work on windows (don't know if that's possible)
* use the php5 built-in tidy module if available, that seems to allow a fine-grained config on which tags are allowed as well (see http://www.php.net/manual/en/ref.tidy.php)