function Scheduler(async) {
		this._async = async;
		this._running = false;

		this._queue = this;
		this._queueLen = 0;
		this._afterQueue = {};
		this._afterQueueLen = 0;

		var self = this;
		this.drain = function() {
			self._drain();
		};
	}
Async task scheduler
@param {function} async function to schedule a single async function
@constructor