Add flag to register RSVP.on callbacks

After brainstorming with @stefanpenner:

```javascript

window.__PROMISE_INSTRUMENTATION__ = {

  created: function(e) {
    // promise was created
  },
  fulfilled: function(e) {
    // promise was fulfilled
  }
  //...
};
```

This instructs RSVP to set up `RSVP.on` listeners on each of the provided callbacks.  That way we ensure that we don't miss any event.  We can inject the promise assembler later and collect what was triggered.