Setup Checkpoints controller
Add missing migration for creating Lesson

DERP? How did this go missing?
Create CheckpointCompletions
Rename checkpoint content to body
Update ember-rails, ember, and ember data
User can complete a checkpoint
Use ActiveModel Adapter and Serializer
Add RVM config files
Checkpoint completions must be unique per checkpoint, per user
Lessons have many checkpoint completions
Show checkpoints and allow them to be marked as complete
Update Ember and Ember Data
Async load Checkpoint Completions
Fix weirdness when completing a Checkpoint where UI wasn't updating