Add callback trigger function

I find myself constantly wanting to trigger an event on execution of a callback (eg. Trigger an event on an ajax response).

```javascript
this.doSomething = function() {
  $.get('/hello', $.proxy(function() { 
    this.trigger('world', arguments); 
  }, this)); 
}
```

Not sure if other people are doing this or a lot or just myself! This pull request adds a new `thenTrigger()` method to component.js to simplify the process of triggering an event on the calling component on execution of a callback.

eg.
```javascript
this.doSomething = function() {
  $.get('/hello', this.thenTrigger('world');
}

this.on('world', function(e, res) {
  // Arguments are passed through if needed.
  console.log(res);
});
```

Is this something that would help others?