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.

@hjdivad & @stefanpenner