_getInstance = function(type, driverName)
	        {
	            var d, i;
	            driverName = (driverName? driverName : _self.defaultDriver).toLowerCase();

	            if(driverName in _drivers[type])
	            {
	                d = _drivers[type][driverName];
	                i = new d();
	                i.driverName = driverName;
	                return i;
	            }
	            throw 'Driver ' + name + ' not found';
	        }
Create a tween or timeline object of the specified driver. If driverName is not given, it fallbacks to default driver

@param {string} 'tween' or 'timeline'
@param {string} [driverName] - one of the registered driver's name
@returns {object} tween or timeline object