code: function (type, content, link, date) {
                      var notification = {
                          id: ++lastId,
                          type: type,
                          content: content,
                          link: link,
                          date: date || new Date()
                      };
  
                      notificationsState.value().push(notification);
                      notificationsState.save();
  
                      return notification.id;
                  }
docs: @ngdoc function
  @name w20CoreNotifications.service:NotificationHistoryService#addNotification
  @methodOf w20CoreNotifications.service:NotificationHistoryService
  @param {String} type The notification type (info, warn or alert).
  @param {String} content The notification content.
  @param {String} link The notification link (optional).
  @param {Date} date The notification date (optional).
  @returns {Number} the notification id
  
  @description
  
  Add a new notification