Add support for react animations

[basvandijk]

I would to use the ReactCSSTransitionGroup addon in my blaze-react app.

See: http://facebook.github.io/react/docs/addons.html

I tried changing require('react') to require('react/addons') and splicing in a ReactCSSTransitionGroup tag in my Html. Unfortunately I got an error from the invariant function that ReactCSSTransitionGroup couldn't be found.

Any ideas how to use addons and third-party plugins in blaze-react?