Introducing mw.libs, moving JpegMeta into it.

Introducing mw.libs, moving JpegMeta into it.
* Reverts/re-do's r88794 and r88796.
** mediawiki.util.js no longer extends itself but is defined once.
** mediawiki.util.jpegmeta no longer extends mw.util but is installed as an object property of mw.libs
* Empty placeholder defined in mediawiki.js
* Removed the redundant 'mw' argument from the IIFE around mediawiki.libs.jpegmeta.js

* Fixed all usages in /mediawiki/trunk/*
** http://toolserver.org/~krinkle/wikimedia-svn-search/view.php?id=205&hash=ddc0908eef111558816c9fe1c775f7c1