Add EventTarget based error handling.

Adds the ability to register multiple error handlers via `RSVP.on`:

```javascript
RSVP.on('error', function(event) {
  console.assert(false, event);
});
```

Deprecates usage of `RSVP.configure('onerror', yourCustomFunction);`.

This also moves the `configure` function into `config.js` (which seems
more appropriate).