Avoids duplicated registration of event handler/Optional deregistration of event handlers

[Eduardo-M-Cavalcanti]

Modification to method $on, avoiding duplicated registration of event handler.

[Eduardo-M-Cavalcanti]

There is a former push request with optional deregistration of event handlers. This one allows deregistration of multiple handlers in the same call. In line with $on, that allows the the registration of multiple handlers in the same call