Add CSSTransitionGroup to addons

Stealing part of #1059.

Author: zpao

src/browser/ReactWithAddons.js

@@ -27,13 +27,15 @@
 
 var LinkedStateMixin = require('LinkedStateMixin');
 var React = require('React');
+var ReactCSSTransitionGroup = require('ReactCSSTransitionGroup');
 var ReactTransitionGroup = require('ReactTransitionGroup');
 
 var cx = require('cx');
 
 React.addons = {
   classSet: cx,
   LinkedStateMixin: LinkedStateMixin,
+  CSSTransitionGroup: ReactCSSTransitionGroup,
   TransitionGroup: ReactTransitionGroup
 };