Use window.performance.now if available, otherwise fallback to Date.now polyfill

This PR adds a `TWEEN.now` method which returns `window.performance.now` if available.

This is a timestamp from the site load with submillisecond precision. This helps a little with rounding issues with regards to 16.7 ms frames.

If the method isn't available, then it falls back to `Date.now` or the old polyfill.

Let me know if something needs to change. Tests pass. Not sure if I got the style-guide-y stuff right.