Use triggerMethod on child view if available

This PR fixes #1488.  It abstracts out the logic a collection view uses for calling `triggerMethod` on a child view.  If the child view implements `triggerMethod`, then it calls that.  Otherwise, it calls its own implementation on the child view.

I cleaned up one of the specs that was kind of related, but we probably want specs that actually verify that the correct `triggerMethod` is called.

Please look over this to make sure this satisfies the requirements of #1488 and isn't too crazy :sleeping: 
yeah, I think it'd be better just to use
```js
_.each(this._bufferedChildren, _.bind(this._triggerMethodOnChild, this, 'show');
```