Setting Motion props in onRest callback won't start new animation

I'm not sure if this is a bug, but it bit me today.

In my code, a Motion onRest callback triggers a re-render of my React component, which sets new props for that Motion component, which are then ignored because the animationID has not yet been set to null [here](https://github.com/chenglou/react-motion/blob/master/src/Motion.js#L105) which means no new animation is created [here](https://github.com/chenglou/react-motion/blob/master/src/Motion.js#L214).

Expected behavior: a new animation is created, Motion works as usual.

Possible fix: call onRest after animation state has been reset.

Alternate fix: in development, React-Motion could detect it's inside an onRest callback, and print warnings if calls are made that can't be handled at that time.

There is a workaround [here](https://github.com/chenglou/react-motion/issues/224#issuecomment-218277983) using a setTimeout with 0.

Start in src/Motion.js around lines 105 and 214, tracing how animationID is reset relative to the onRest callback. Reproduce the callback-triggered prop update described in the issue, then verify that a new animation starts without the setTimeout workaround. The issue does not mention a test file.

Written by the indexing model from the issue text.