docs: function authorize(storagePath,
                     clientSecrets,
                     scopes,
                     authCallback) {
    const oauthClient = createOauthClient(storagePath, clientSecrets);
    const url = oauthClient.generateAuthUrl({ access_type: 'offline', 
                                            scope: scopes });
    
    authCallback(url, function(code, completion) {
      oauthClient.getToken(code, function(err, token) {
        if (err) {
          completion(new Error('Invalid token entered'));
        } else {  
          try {
            fs.writeFile(path.join(storagePath, clientSecrets + '.token'), JSON.stringify(token)); 
            completion();
          } catch (err) {
            completion(new Error('failed to write token to file:' + err));
          }
        }
      });
    });
  }