add method getUrlAuthorization

添加一个生成UrlAuthorization的方法。
是的， 错了。应该是秒。

```
OSS.prototype.getUrlAuthorization = function (bucket, name, expireSec) {
  var expireDate = Date.now()/1000 + expireSec;
  var basicString = crypto.createHmac('sha1', this.accessKeySecret);
  var str = 'GET\n\n\n' + expireDate + '\n/' + bucket + '/' + name.trim();
  basicString.update(str);
  return 'OSSAccessKeyId=' + this.accessKeyId + '&Expires=' + expireDate + '&Signature=' + basicString.digest('base64');
}

```