Added User <=> Courses association

* Added a notion of fixtures. See spec_helper
* Not all tests pass yet
Added Course lookup by user

* /api/users/:user_id/courses now lets you look at all courses owned by
  a particular user
* POST to /api/courses is now invalid for creating
* POST to /api/users/:user_id/courses is now valid for creating
* user_id field now is a member of a json serialized Course
* added 'user_two' and 'course_three' to test fixtures
Some documentation fixes for course->user
Database seeds now incorporate user to course association

* Also made user and user_id attr_accessible in course
Merge remote-tracking branch 'origin/api-users-courses'

Conflicts:
	app/controllers/api/courses_controller.rb
	app/models/course.rb
	app/serializers/course_serializer.rb
	db/schema.rb
	db/seeds.rb
	spec/controllers/api/courses_controller_spec.rb