use classList API to add/remove CSS class

Hi YUI Team,

I'm pull-requesting you, my implementation of the classList API, for modern browsers which can handle it. For old ones, it use the actual mecanism.

I'm refering to this ticket http://yuilibrary.com/projects/yui3/ticket/2528188 and this Paul Irish video http://www.youtube.com/watch?v=hZJacl2VkKo about the performance of this API.

Julien