Use CSSCore instead of jQuery in TimeoutTransitionGroup

This isn't the correct way to add multiple classes – addClass takes only one class at a time. See the corresponding lines in ReactCSSTransitionGroupChild's source:

https://github.com/facebook/react/blob/0db323e42adae6506cc8bec3dc094dc81c80612c/src/addons/transitions/ReactCSSTransitionGroupChild.js#L92-L94