KeepAlive now supported

- KeepAlive notifications are now supported. KeepAlive notifications make sure that the client 
  is aware that a subscription (and its monitored items) is still alive even though no notifications
  were send lately (since no data had changed or no events were sent).

- BaseNotification is now known as MonitoredItemNotification, to clarify that it is only a 
  superclass of DataChangeNotification and EventNotification, and not of other notifications such as
  the new KeepAliveNotification.