`React.addons` is where we park some useful utilities for building React apps. **These should be considered experimental** but will eventually be rolled into core or a blessed utilities library:
-`ReactTransitions`, for dealing with animations and transitions that are usually not simple to implement, such as before a component's removal.
-`ReactLink`, to simplify the coordination between user's form input data and and the component's state.
-`classSet`, for manipulating the DOM `class` string a bit more cleanly.
