Add instrumentation.

```js
function listener (event) {
  event.guid            // guid of promise
  event.childGuid       // guid of child promise (for chained via `then`)
  event.eventName       // one of ['created', 'chained', 'fulfilled', 'rejected']
  event.detail          // fulfillment value or rejection reason, if applicable
  event.label           // label passed to promise's constructor
}

RSVP.Promise.on('created', listener);
RSVP.Promise.on('chained', listener);
RSVP.Promise.on('fulfilled', listener);
RSVP.Promise.on('rejected', listener);
```

Events are only triggered when `RSVP.Promise.instrument` is true, although
listeners can be registered at any time.