[WIP] Use Order model

This PR refers to: https://github.com/Sylius/SyliusSalesBundle/issues/30 The aim is to delete the duplication between Cart and Sales bundle.

First step is updating Order model to be able to deal with Cart system. This is done by https://github.com/Sylius/SyliusSalesBundle/pull/31 and *this current PR depends on it*.

Second step is using Order model from CartBundle. This is what this PR does.

Todo:
- [x] I just modified all Cart and CartItem instances to Order and OrderItem
- [x] Re-add Cart and CartItem models to keep Cart specific logic in this bundle and leave SalesBundle clean
- [ ] Use dbu's solution to remove Cart entity and solve the model extending issue