Make wp-rest-jsonapi-serializer more WordPress idiomatic

[taras]

Let's continue the conversation from oskarrough/ember-wordpress#23 in this issue.

Here is some recommended reading to further this conversation http://v2.wp-api.org/extending/modifying/ and https://make.wordpress.org/core/2016/05/03/a-data-schema-for-meta/

[daltonrooney]

Here's some more up to date information on the register_meta function, which changed a bit in the last WordPress release cycle: https://make.wordpress.org/core/2016/07/20/additional-register_meta-changes-in-4-6/

As far as I can tell, setting show_in_rest as true does not actually add the field to the REST API, it simply makes the field available for registration. You still need to register_rest_field to add a custom field to a post type endpoint.