简单ajax封装类
function ajax(getorpost,url,data,success,failure){
var xmlHttpRequest;
if(window.ActiveXObject){
xmlHttpRequest=new ActiveXObject("Microsoft.XMLHTTP");
}else{
xmlHttpRequest = new XMLHttpRequest();
}

var statechange = function(){
if(xmlHttpRequest.readyState==4){
if(xmlHttpRequest.status==200){
success(xmlHttpRequest.responseText);
}else if(xmlHttpRequest.status==404){
failure(xmlHttpRequest.responseText);
}
}
}
xmlHttpRequest.onreadystatechange = statechange;
if(getorpost=="post"){
xmlHttpRequest.open(getorpost,url,true);
xmlHttpRequest.setRequestHeader("content-length",data.length);
xmlHttpRequest.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
xmlHttpRequest.send(data);
}else if(getorpost=="get"){
xmlHttpRequest.open(getorpost,url,true);
xmlHttpRequest.send(null);
}