创建xhr对象实现浏览器全兼容

        
  function createXmlHttp() {//创建xhr对象
                var xhobj = false;
                try {
                    xhobj = new ActiveXObject("Msxml2.XMLHTTP"); // ie msxml3.0+
                } catch (e) {
                    try {
                        xhobj = new ActiveXObject("Microsoft.XMLHTTP"); //ie msxml2.6
                    } catch (e2) {
                        xhobj = false;
                    }
                }
                if (!xhobj && typeof XMLHttpRequest != 'undefined') {// Firefox, Opera 8.0+, Safari
                    xhobj = new XMLHttpRequest();
                }
                return xhobj;
            }