An accurate, multienvironment timer

##### accurate_timer uses requestAnimationFrame() and performance.now(), defaulting to setInterval if it's used in a non-browser or older environment.

Based on my own (unscientific) measurements, the timer in a node.js environment tends to be accurate to about 3/1000ths of a second. In a browser environment it's a little less than 1/1000ths of a second.

Usage

* Create a timer object:
``js
var timer = new Timer(, );
`

The callback will be called every interval milliseconds.

* start / stop the timer:
`js
timer.start();
timer.stop();
``

All done! I hope you enjoy :blush: