Add >2 levels of 'belongs_to' nesting in URLs

This enables URLs of the form /user/X/posts/Y/comments/Z and /user/X/comments/Z, at the same time. Simply use

```ruby
class CommentsController < ResourceController
  belongs_to :post, :user
end
```

and add the appropriate resources routes in `routes.rb`, and all URL helpers will magically "just work". (object_url, etc). Note that you need to specify `:post` before `:user` because `:user` is contained in both URLs. Otherwise the URLs will be incorrect.