Create categories

* Use a self-referential relationship to create a tree structure.
* Fix Zeus-related issue with Shoulda Matchers not being loaded during
  individual spec file runs.
* Apply style guideline "Prefer single quotes for strings."
* Associate categories with accounts.
* Use RSpec `expect` syntax.

https://trello.com/c/TkIOAdRV
Remove self-referential nesting of categories
Enforce uniqueness of category name by account

* Improve readability of sections of model and model spec code with
  comments defining the section.
* Use unique index for data integrity as suggested by
  http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-validates_uniqueness_of-label-Concurrency+and+integrity