Add API Builder: ActiveModel::Serializers.

> `ActiveModel::Serializers` encapsulates the JSON serialization of objects. 
Objects that respond to read\_attribute\_for\_serialization 
(including `ActiveModel` and `ActiveRecord` objects) are supported.
>
> Serializers know about both a model and the `current_user`, so you can
customize serialization based upon whether a user is authorized to see the
content.
>
> In short, **serializers replace hash-driven development with object-oriented
development.**