Api Delete For [almost] All Models

* Implemented delete for models
  * Courses
  * Enrollments
  * Users
  * Quizzes
  * Problems
  * Lessons

This adds an additional API endpoint to each of these controllers. The URL is the same as #get or #update, but the HTTP action is DELETE instead of GET or PUT.

The endpoints return a 200 success code, and no assumptions may be made about the body of the output.

Changes have been made to
* All respective controllers for those models
* All test suites for those controllers
* Models to add destruction dependencies between them. That is, if a Course gets deleted, all of its associated Lessons and Quizzes should be deleted.
* Tests for those destruction dependencies are in spec/model

@edsamson