http://www.cnblogs.com/dragonpig/archive/2011/01/15/1936512.html
var array = [1,2,3,4];
for(var i=0,len=array.length;i<len-1;i++){
var pos = i + Math.floor((len - i)*Math.random());
var tmp = array[pos];
array[pos] = array[i];
array[i] = tmp;
}
alert(array.join(', '));