Serialize embedding in root deeply nested associations

Closes #444
Allow to pass options to associations

Closes #331
When runnin w/in Rails, make available url_helpers

This technique ties this usage to Rails, which but makes the helpers
directly accessible w/in any instance of ActiveModel::Serializer.
Pass url_options on to associations

So that they too can generate URLs via url_helper
Use Category for nesting hypermedia test data

The Hypermedia* models and serializers didn't were a bit nonsensical,
and thus hard to reason about. Category seems to fit the domain of the
other test models.