Add not loading components on AJAX calls

Allow setting whether to register CSS and/or JS components on AJAX calls.

This is particularly useful when using the *CController::renderPartial* function with the *processOutput* argument set to *true*.

Some history:

http://stackoverflow.com/questions/9397149/yii-unload-bootstrap-plugin-on-ajax-request

http://www.yiiframework.com/forum/index.php/topic/34996-yii-ajax-duplicate-clientscripts/page__p__168372#entry168372

However neither approach works in all cases (thought the second provided the inspiration for this patch).

Thanks!