使用动态重写，实现JS的d单件模式(singleton)

支持FF/IE/OP/SF
js真的太灵活了。

<script type="text/javascript">
    function singleton() {
        function s() {
            
        }
        var i = new s();
        singleton = function() {
            return singleton.instance;
        }
        singleton.instance = i;
        return singleton();
    }
    var s1 = new singleton();
    var s2 = new singleton();
    var s3 = new singleton();
    alert(s1 === s2);
    alert(s1 === s3);
  </script>