code: func (oauth *Oauth) GetUserAccessToken(code string) (result ResAccessToken, err error) {
  	urlStr := fmt.Sprintf(accessTokenURL, oauth.AppID, oauth.AppSecret, code)
  	var response []byte
  	response, err = util.HTTPGet(urlStr)
  	if err != nil {
  		return
  	}
  	err = json.Unmarshal(response, &result)
  	if err != nil {
  		return
  	}
  	if result.ErrCode != 0 {
  		err = fmt.Errorf("GetUserAccessToken error : errcode=%v , errmsg=%v", result.ErrCode, result.ErrMsg)
  		return
  	}
  	return
  }
docs: // GetUserAccessToken 通过网页授权的code 换取access_token(区别于context中的access_token)