Basic models for course and lesson

* Controllers api/course api/lesson
* Models for course and lesson
* Migrations for course and lesson
* Gem for serialization
* Gem for Model documentation
Added serializers for course and lesson

* Fixed bug on serializer that nested the json payload within a sub key
Lesson index correctly filters by course
Made api endpoints more robust

* Error messages now provided if resources not found
* Lessons endpoints now enforce that course id matches
* Added seeds.rb to seed the database
* Added non null constraints on course and lesson fields