Remove mediator pub-sub, delegate EventBroker to Backbone.on-off

As per discussion in gh-334:

1. Backbone implemented global events in 0.9.9. Would be nice to have compat with this. A while after bb-1.0, users will be confused that our `publishEvent` doesn’t work with BB global events.
2. Usage of `mediator.*` pub-sub is discouraged in favour of Event Broker’s `this.publishEvent` etc methods.
3. Our `EventBroker` preserves context of `this`, which means If we will switch to `mediator.subscribe = Backbone.on`, **nothing** will change for those who use `EventBroker` for pub-sub.