gamejs.time: use requestAnimationFrame()  instead of setInterval()

gamejs.time uses reqAniFrame internally if available; the API
has not changed: you can still pass the `fps` you want the function
to be called and the thisObj.

However, you are encoureged to use `gamejs.time.interval()` since
the horribly named `gamejs.time.fpsCallback()` is now deprecated.
fix some intra-jsdoc links
add Sprite.groups()