_tick = function()
	    {
	        var now = _now();
	        var i = 0;
	        while(i < _callbacks.length)
	        {
	            var entry = _callbacks[i];
	            var call = (!entry.time || now - entry.now - entry.time >= 0);
	            if(call && entry.time)
	            {
	                // remove expired callback
	                _callbacks.splice(i, 1);
	            }
	            else
	            {
	                i++;
	            }
	            if(call)
	            {
	                entry.callback.apply(entry.scope, entry.params);
	            }
	        }
	        // ticker is turned off when the callbacks list is empty
	        if(_callbacks.length)
	        {
	            _requestAnimationFrame(_tick);
	        }
	    }
Ticker object used to emulate delay, progress callbacks, async calls.
It uses RequestAnimationFrame when available, with fallback to setTimeout.
We instantiate a single Ticker in Tweene.ticker
@class