Ignore transition end events from nested nodes (fixes ReactCSSTransitionGroup behaviour)

This commit fixes pretty common problem: when you have nested node with CSS transition (non `ReactCSSTransitionGroupChild`), end event bubbles up to the parent that leads to prematurely node removal.

E.g modal window (wrapped into `ReactCSSTransitionGroupChild`) going to move up and then disappear after pressing "close window". But right after window (with that button) move a bit up button gets mouse out event, that leads to background transition (on hover → normal state) that leads to transition end event. End event bubbles up and modal window get's it. Boom, modal window removed earlier than it should be.