[WIP] Make Order model Cart friendly

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 what this PR does.

Todo:
- [x] I just included the method of Cart and CartItem that didn't appear in Order and OrderItem. So that CartBundle can rely on Order and OrderItem without modification;
- [x] I modified a bit Order model constructor. I set "number" attribute to `uniqid(...)` because it cannot be null in db, but it's CartBundle that create the order instance now. I don't know if we can move this logic from Sales to Cart bundle, it makes sense to define a unique number to a Cart, why not.
- [x] Set Order.number nullable for now
- [x] Import CartBundle specs
- [ ] Delete OrderBuilder ?
- [x] Simplify Order: remove `totalItems` and `totalQuantity` attributes? And keep the associated getters that calculate the right result
- [x] Simplify Order: add a `state` attribute to include all possible states: cart, cart locked, order unconfirmed, order confirmed (any other?)