shallow nesting

Hi Eliot, I think this is all working. take a look!
Try deleting a comment with the app from the shallow-nesting branch. There may be a problem as `posts` on this line doesn't exist. 

`find` is available as a class method on `Post` and all other model classes (e.g. see the old line 18 in this comments_controller which used the `Topic.find(...)` class method.)