Add mixins for supporting dirty/saved states of parent/embedded records

- Embedded in model mixin marks parent record dirty if embedded record becomes dirty
- Mark embedded records clean after save
- Refactor adapter/serializer, test load order, Ember config (setupStore) for testing
- Add test for embedded model adapter 404 response
- Add tests for both embedded belongsTo and hasMany relations
- Update Makefile, Add notes to README, update runner.html