Initial stab at breaking math/texvc out to Math extension.

Initial stab at breaking math/texvc out to Math extension.

* (bug 14202) $wgUseTeX has been superseded by the Math extension. To re-enable
  math conversion after upgrading, obtain the Math extension from SVN or from
  http://www.mediawiki.org/wiki/Extension:Math and add to LocalSettings.php:
  require_once "$IP/extensions/Math/Math.php";

This is an initial stab, and a few things remain to be cleaned up:
* messages need to be moved from core to extension
* MW_MATH_* constants should be moved to the extension from core
* old back-compat math names interfaces using those constants should be removed from message files
* classic edit toolbar's math button should be added from the extension (or else dropped) -- currently there's not a clean hook, but could do it by JS
* couple of things like the 'armourMath' function on Language & LanguageConverter may want to be redone just as an unconditional, if that's simpler.

Setting $wgUseTeX alone will no longer have any affect. The var's still there for the moment as a few bits still need to be fully moved out from core.