Improved Callback Messages for Cartago

[KaiTries]

An agent is able to focus on an artifact and can use operations if available. If an agent is focused on an artifact he will receive messages when its observable properties have changed. Currently the message received is just a simple string representing the property and its value: somObservableProperty(itsCurrentValue)

Since we have all necessary information to actually send a more sophisticated / in-depth message there is no reason we should not be doing that. To be in line with the other responses it should be some valid rdf response.