Added a Clock abstraction.
Added a Clock abstraction.

This saves having to do this same math in a zillion places throughout
libraries.

longtaoge/playn,tinkerstudent/playn,tinkerstudent/playn,playn/playn,ruslansennov/playn2,playn/playn,rachelharvey/playn,longtaoge/playn,ruslansennov/playn2,playn/playn,thecocce/playn-1,thecocce/playn-1,rachelharvey/playn