Always CSS transitions when available

This makes a huge difference in perceptual animation quality from ~10-20 FPS on a 1.7Ghz Core i7 to 60FPS.

I'm working on a few more changes – I think we can avoid a number of invalidate/recalc cycles which would be a big win on mobile – but wanted to send this over because it's a small change and it might a good time to review the animation strategy now that CSS transition support is [well over 80% and climbing](http://caniuse.com/#feat=css-transitions) as people upgrade away from IE 8/9.