[FEATURE] Dispatcher disable/enable events on execution

Sometimes, I would like to enable/disable specific events for a moment.

This PR adds two public methods to the EventDispatcher `addHandler` and `removeHandler` to be used during execution.

One of the case is to disable user events during a promise resolution, I also [open the discussion](http://discuss.emberjs.com/t/blocking-actions-and-user-events-until-promise-resolution/4936) which would like to be considered.