* Ended the role of $wgLinkCache in link updates. Instead, links (and related entities) are registered in the ParserOutput object during a parse. The LinksUpdate constructor now takes a ParserOutput object as a parameter. $wgLinkCache is still used, but only as a cache of article IDs.
* Because the link list is now saved and restored in the parser cache, meta tag keywords now work on parser cache hits. Some refactoring took place in this area.
* Rendering of the HTML for category links has moved from Parser to OutputPage.
* Did some general pottering around in Article.php, such as allowing an Article object to be created with a specified revision ID, thereby optionally removing the dependence on $wgRequest. Not used at the current time.
* A few documentation tweaks.
