* Added WikiPage::getParserOutput() and changed Article::getParserOutput() to use it

* Added WikiPage::getParserOutput() and changed Article::getParserOutput() to use it
* WikiPage::getParserOutput() requires a ParserOptions object (and optionally the revision ID) instead of an User object, removes an hidden dependency on $wgLang. For this reason, WikiPage::isParserCacheUsed() now also uses a ParserOptions object instead of an User object (doesn't change anything in the code except the variable name and it's not called in extensions)
* Moved PoolWorkArticleView to WikiPage.php and added an entry in the AutoLoader and moved output-related stuff directly in Article::view() so that in can be shared with WikiPage::getParserOutput() (removes code duplication, etc.)
* Added the revision ID to the PoolCounter key so that it knows which revision is being parsed and doesn't wait for another parse operation with same options but different revisions
* Removed Article::doViewParse(), Article::tryDirtyCache() and Article::getOutputFromWikitext() since they are now integrated in PoolWorkArticleView and Article::view() and there are no callers in extensions. This also fixes a bug since Article::doViewParse() will get another ParserOptions instance with special options set in Article::view() not be repercuted.
* Updated DifferenceEngine to use the new system
* Updated docs/memcached.txt to correct method names