Adds ActiveModelSerializers to define JSON output on models.

* Adds Active Model Serializers
* Configures Active Model Serializers to not use root element.
Edits API controllers JSON output

* Edits API routes.
* Adds base serializers for Customer, Payment, Purchase, LineItem.
* Relates User to Payment, Purchase through Customer.
* Edits API controller responses to scope by User related records
  depending on provided authentication token.