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: 
jshint didn't like the length of the single line.  if-else or we could grab the source receiver instead?

```js
var triggerMethodSource = _.isFunction(child.triggerMethod) ? child : this;
triggerMethodSource.triggerMethod.apply(child, Array.prototype.slice.call(arguments, 1));
```