Use Polymer/core-animation instead of manual CSS animations.

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](http://www.polymer-project.org/docs/elements/core-elements.html#core-animation) works in IE9, so I updated <paper-elements/> to use it.

(Sorry for the apparently huge changes to the CSS, all I did was change two comments and remove the CSS animations...  git's diffing tool disagrees.)