js toFixed 方法重写，兼容负数

Number.prototype.toFixed = function (s) {
        var that = this, changenum, index;
        if (this < 0) {
            that = -that;
        }
        changenum = (parseInt(that * Math.pow(10, s) + 0.5) / Math.pow(10, s)).toString();
        index = changenum.indexOf(".");
        if (index < 0 && s > 0) {
            changenum = changenum + ".";
            for (var i = 0; i < s; i++) {
                changenum = changenum + "0";
            }
        } else {
            index = changenum.length - index;
            for (var i = 0; i < (s - index) + 1; i++) {
                changenum = changenum + "0";
            }


        }
        if (this < 0) {
            return -changenum;
        } else {
            return changenum;
        }
    }