code: function generate_google_oauth2_url () {
    var access_type = (OPTIONS.access_type) ? OPTIONS.access_type : 'offline';
    var approval_prompt = (OPTIONS.approval_prompt) ? OPTIONS.approval_prompt : 'force';
    var url = oauth2_client.generateAuthUrl({
      access_type: access_type, // set to offline to force a refresh token
      approval_prompt: approval_prompt,
      scope: OPTIONS.scope // can be a space-delimited string or array of scopes
    });
    return url;
  }
docs: getGoogleAuthURL creates a url where the user is sent to authenticate
  no param
  @returns {String} url - the url where people visit to authenticate