Question controller: tighten up the edit method, change specs to use context instead of describe where appropriate
clean up comments controller and add specs
Changes to user model: made REGISTRATION_KEY a constant, corrected name of association 'answers' to 'assigned_questions'
fix two bugs in the admin user controller
update user controller for changed association name 'assigned_questions', remove unused actions
added new controller specs, changed user factory, adding 'admin' and removing admin status from 'instructor'