code: function OauthAuthenticator(options) {
    Authenticator.call(this);
    if (typeof(options.credentials) === 'string') {
      this.credentials = {
        'access_token': options.credentials
      };
    } else {
      this.credentials = options.credentials || null;
    }
    this.flow = options.flow || null;
    this.app = options.app;
  }
docs: Creates an authenticator that uses Oauth for authentication.
  
  @param {Object} options Configure the authenticator; must specify one
  of `flow` or `credentials`.
  @option {App}           app           The app being authenticated for.
  @option {OauthFlow}     [flow]        The flow to use to get credentials
  when needed.
  @option {String|Object} [credentials] Initial credentials to use. This can
  be either the object returned from an access token request (which
  contains the token and some other metadata) or just the `access_token`
  field.
  @constructor