add ES6 support

interval.js

export default function timer(fn, duration) {
	this.baseline = 0

	this.run = function(exec){
		if(!this.baseline){
			this.baseline = Date.now()
		}

		exec && fn()

		this.baseline += duration

		var end = Date.now()
		var nextTick = duration - (end - this.baseline)

		if(nextTick < 0) {
			nextTick = 0
		}

		var self = this
		this.timer = setTimeout(() => self.run(true), nextTick)
	}

	this.stop = function(){
		this.baseline = 0
		clearTimeout(this.timer)
	}
}