code: function(username, password, callback) {
      return this._postParams({
        grant_type : "password",
        username : username,
        password : password,
        client_id : this.clientId,
        client_secret : this.clientSecret,
        redirect_uri : this.redirectUri
      }, callback);
    }
docs: OAuth2 Username-Password Flow (Resource Owner Password Credentials)
  
  @param {String} username - Salesforce username
  @param {String} password - Salesforce password
  @param {Callback.<TokenResponse>} [callback] - Callback function
  @returns {Promise.<TokenResponse>}