属性扩展

// 属性扩展
        // @param isCover 是否覆盖已有属性  默认true
        this.extend = function(destination, source, isCover) {
            var isUndefined = this.isUndefined;
            (isUndefined(isCover)) && ( isCover = true);
            for (var name in source) {
                if (isCover || isUndefined(destination[name])) {
                    destination[name] = source[name];
                }
            }
            return destination;
        };