Create categories

* 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.
* 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

https://trello.com/c/TkIOAdRV