Make API clearer re: when spring is at rest
The public API was confusing with regards to when the spring at rest, and didn't differentiate between "is animating" and "is at rest".
- isAtRest and isAnimating are public getters
- onActive -> onStart, onAtRest -> onStop
- Added test for stop/onStop
Test Plan: Run the tests! :)
Reviewed By: appsforartists
Differential Revision: http://codereview.cc/D3307