* Added revision's timestamp to OutputPage along with revision ID; avoid a DB hit...

* Added revision's timestamp to OutputPage along with revision ID; avoid a DB hit in Skin::lastModified() when showing parser cache's content. This changed with the removal of $wgArticle in Skin since now it's a different WikiPage object and thus WikiPage::setTimetstamp() call is useless (but still kept).
* Added ParserOutput::(get|set)Timestamp() and the $mTimestamp member; avoid messing with isset()