A user model can be joined with with a course

Courses can see which users have joined
Updated courses_spec to use context in certain places

Wrote stubs for registering a user in a course
POST /courses/:course_id/enrollments/:user_id returns status code 200

POST /courses/:course_id/enrollments/:user_id registers user in course

POST /courses/:course_id/enrollments/:user_id returns 400 if no such course

POST /courses/:course_id/enrollments/:user_id returns 400 if no such user
Stubbed out specs for GET /courses/:course_id/enrollments

GET /courses/:course_id/enrollments returns status code 200

GET /courses/:course_id/enrollments returns json representation of course's users

GET /courses/:course_id/enrollments returns 400 if no such course
Stubbed specs for GET /users/:user_id/courses

GET /users/:user_id/courses returns status code 200

GET /users/:user_id/courses returns JSON representation of users's courses

GET /users/:user_id/courses returns 400 if no such user
Stubbed out specs for DELETE /courses/:course_id/enrollments/:user_id

DELETE /courses/:course_id/enrollments/:user_id returns 200

DELETE /courses/:course_id/enrollments/:user_id removes the student from the course

DELETE /courses/:course_id/enrollments/:user_id returns 400 if no such course/user