* (bug 29140) FirePHP debugging assist extension

* (bug 29140) FirePHP debugging assist extension

FirePHP is an extension to Firebug, an awesome debugging extension to Firefox.
It allows pulling debug log data in from your PHP script via HTTP headers, which then get displayed in Firebug's console along with errors, warnings, and AJAX hits from the web page.

Added 'Debug' hook which gets called from wfDebug() and wfDebugLog() to take this; note that a few lines of output won't make it to FirePHP as they are output either before we've done all plugin initialization, or after we've flushed output and can no longer add HTTP headers.

BSD-licensed FirePHPCore library from firephp-libs commit aff25803a3ff460b2797:
https://github.com/cadorn/firephp-libs/blob/aff25803a3ff460b2797/packages/core/lib/FirePHPCore/FirePHP.class.php

There's not a lot of fancy integration; everything's just output as a 'log' line.
You can use $wgFirePHP global (or call FirePHP::getInstance() yourself) to get direct access to FirePHP's fancier features in test code.