Ensure `listenToOnce` is only called once.

It allows you to [stop listening](https://github.com/jashkenas/backbone/blob/master/backbone.js#L135-L141) to the callback. It mirrors `once`'s [behavior](https://github.com/jashkenas/backbone/blob/master/backbone.js#L96-L100).

```javascript
function cb() { console.log('callback'); }

// without `_callback` property
obj.listenTo(obj, 'event', cb);
obj.stopListening(obj, 'event', cb);
obj.trigger('event'); // "callback" logged
```