use updated restpack-serializer gem

now requires explicity loading of serializers via initializer and protected_attributes gem

https://github.com/RestPack/restpack_serializer/pull/92
Merge pull request #168 from camallen/restpack_gem_upgrade

use updated restpack-serializer gem
Add UserEnqueuedSubjects denormalization

Add after_create and after_save callbacks to Classification
model to manage denormalization
Add queued sort to subjects controller
Refactor classifications model

Use better boolean method naming, remove state enum
Remove UserEnqueuedSubject model when no more subject_ids
Add SubjectSelector class