These typings are mostly copy-pasted from react-addons-update. Because this library
strives to be a drop-in replacement for react-addons-update, that seems appropriate.
I did not define it in terms of react-addons-update (i.e., didn't import
react-addons-update and then re-export it) because it won't necessarily track React,
so it should remain separate. Plus, it adds a couple functions.
