Tween mixin

This addresses GH-4.

An alternative idea for cleaning up tweens would be to catch errors when calling `setState` and silently kill the tweens on them. (This has some negative troubleshooting and potentially performance implications.)

Checking isMounted doesn't seem to work because (I think) we want to allow the state of unmounted components to be changed *if they are mounting.* There doesn't seem to be a way to check that directly (i.e. without a mixin), however.