Use the conventions that https://github.com/Eduki/eduki/blob/master/app/controllers/api/courses_controller.rb uses in conjunction with the superclass, https://github.com/Eduki/eduki/blob/master/app/controllers/api/api_controller.rb.
What this is:
  * A function in Api::ApiController that looks up a user based ID, rendering 404 and returning false on failure, and assigning an instance variable on success
  * A function in Api::UsersController of the same name that just delegates to the super class (see example at bottom of courses_controller)
  * A before_filter at the top of Api::UsersController that references the function