* Changed ParserOptions to store a Language object instead of only a string, avoids...

* Changed ParserOptions to store a Language object instead of only a string, avoids object -> string -> object conversion
* ParserOptions::getUserLang() will still return a string for compatibility, added ParserOptions::getUserLangObj() to get the object
* Added ParserOptions::newFromUserAndLang() and ParserOptions::newFromContext() to easily get a ParserOptions object when a context is available or when someone wants to force the language
* Updated OutputPage and Preferences to use newFromContext() and WikiPage to use newFromUserAndLang()
* ParserOptions::setUserLang() still accepts either a string or a Language object, but changed the calls to pass an object instead of a string
* Changed Parser::getFunctionLang() to return the Language object from ParserOptions when parsing interface messages rather than $wgLang directly and updated the documentation to say that $wgLang should not be used directly (as $wgUser, $wgTitle and $wgRequest)