Event Manager and MessageBroker enhancements

Changes in this pull request includes:

* EventManager: Implementation of unsbuscribe 
* EventManager: Guard against duplicate subscription of same topic-handler
* EventManager & MesageBroker: Change to the message format used to communicate with MessageBroker according to the one agreed with Inkling
* MessageBroker: Closure-ification
* MessageBroker: Implementation of unsubscribe

Also the unit test files (test/spec/eventmanager-spec.js, messagebroker-spec.js) were changed according to the above changes.