function _setInterval(fRunner, nInterval){
        var nTimerId = setInterval(fRunner, nInterval);

        htTimers[nTimerId] = {
            "fRunner"  : fRunner,
            "nInterval": nInterval,
            "bActive"  : true
        };

        return nTimerId;
    }
wrapper of window.setInterval

@param {Function} fRunner
@param {Number} nInterval
@return {Number} nTimerId