Use window.performance.now if available, otherwise fallback to Date.now polyfill
Merge branch 'master' of git://github.com/tdreyno/tween.js into dev

verts commit d6571b01fc0aaf0832a62e01603d7ee3c0d7c9c8, reversing
changes made to c1c86f650453f878797a2e487da7f21d78017de5.
Bringing @mrdoob's latest example change back into dev (sorry about the branch mess :-()