事件监听的封装

function addEvent(ele,type,fun){
    if(ele.addEventListener){    //为非IE
        ele.addEventListener(type,fun,false);
    }else if(ele.attachEvent){   //为IE
        ele.attachEvent('on'+type,function(){
        //IE里面的this指向window故用call改变this指向
        fun.call(ele);           
        })
    }else{
        ele['on'+type] = fun;
    }
}