Updated representation of polymorphic 'belongs_to' associations

Brainstem::Presenter#load_associations! now:
  - represents the polymorphic association attribute name by adding '_ref' postfix 
  - polymorphic 'belongs_to' association is represented as a hash which includes:
    - id: The id of the polymorphic object
    - key: The table name for the class of the polymorphic object
I think support for polymorphic associations is a really good Brainstem improvement, thank you for working on it!

As is, this change will only work when the polymorphic `association(:foo)` is a real polymorphic active record association.  That is, when the database table has foo_id and foo_type columns.  It won't work on a few other cases we support:

```ruby
association(:json_name => "workspaces") { |invoice| Workspace.where(:id => invoice.workspace_cached_ids_for_user(current_user))
association(:method_name)
```

It would be nice if the json_name option could take a hash of class name to json_name for polymorphic association overrides.

We can merge this in, but we should consider it a WIP.