Revert "Use Order model"

This reverts commit 52f651c786aa70558350528cd718ea971fd882dd.
Remove denormalized behavior
Revert "Update specs to use Order model"

This reverts commit 612fc0f9400fcb0c70a751ca96b3daceb200d827.
Re-add Cart models for better separation

/!\ Temporary solution, waiting for Symfony 2.3