Add recipient model

* Adds marginalia development dependency for managing documentation
  generation
* Moves utility functions into utils.clj
* Adds the following models:
    - account
    - recipient
    - subscription
* Embeds documentation for the following models in the source code:
    - subscription
    - customer
    - recipient
* Separates customer, subscription, and recipient tests into their own files
* Adds a test_helper.clj file