Replace the YUI2 code with YUI3.

We could benefit from YUI3 by:

1. load javascript modules dynamically, we don't need to use requires->js anymore
2. YUI 3 has elegant APIs and better performance
3. everything can be module, we require yui.js file once, then we could get everything by ajax, awesome!

In moodle 2.0, we will migrate moodle file picker, filemanager, and commenting system to YUI3 as a module, and try to replace all YUI2 ajax code to new YUI3 style (in javascript-static.js).

In moodle 2.1, we could possiblly create YUI3 modules for each moodle modules, this will make moodle more flexible.

NOTE, a few YUI 2 Widgets don't have YUI3 counterparts yet, however, we can use YUI2 with YUI3 together, Caridy Patino created a YUI2 wraper: http://caridy.github.com/gallery-yui2/, it is very useful.