RSpec and Api::CoursesController Tests

* rspec-rails installed. All tests are now in spec/
* test/ directory destroyed
* Api::CourseController now has tests at spec/api/course_controller_spec
* Api::CourseController now responds with 4XX codes in error cases
Api::LessonsController tests

* Tests for Error codes, tests do not currently pass
* Changed name of Api::CoursesController test to be plural
Lessons now have error responses

* HTTP 404 for id not found
* HTTP 400 for bad params
* All Api::LessonsController tests pass
Forgot to delete file on rename

* this file is now courses_controller_spec.rb
Merge pull request #22 from Eduki/api-courses

RSpec and Api::CoursesController Tests
Approved and code reviewed by me.  Thanks David.