Use <core-animation/> (for IE9 support)

CSS Animations don't work in IE9. It's not just that the animations don't work, but the post-animation callback doesn't get called (so the UI gets stuck in an inconsistent state).

However, Polymer/core-animations works in IE9, so I updated <paper-checkbox/> to use it.

(Previous PR [here](https://github.com/Polymer/paper-checkbox/pull/16), but opening a new one without any LF/CLRF changes and without partial keyframes.