Basic Course/Lessons models and API endpoints

For Courses and Lessons..
* Created migrations for schema
* Created models for both at *app/models/*
* Added API endpoints for both at *app/controllers/api*
* Added Routes for both at *config/routes.rb*
* Added serializers for both at *app/serializers*

Project Wide..
* Added Serializer gem and initializer for it in *config/initializers/serializer.rb*
* Added Annotate gem. run `annotate` in the root to have it automatically add comments to the top of model files based on the schema.

There's a bunch of other files, like stub tests, which are autogenerated by rails model/controller generators. Leave them for now, because we expect to use those exact files later on.