Listener functionality

[MohamadGreatWarrior]

• Each listener will be called with the value of the respective key injected as argument.

• You can register as many listeners as you want for each key.

• When you delete a key its listeners will still be called with null injected as the value for the listener.

• The ability to un-register one listener for specific key with keeping other listeners for that same key.

• The ability to un-register all listeners for specific key.

• The ability to un-register all listeners for all keys.