Dimensions in some entities

Added dimensions in...

* Product |
* Variant |- Purchasable / Persisted
* Cart     |
* CartLine |- Just getters / Not persisted
* Order     |
* OrderLine |- Persisted

When an order is created, all data is loaded and persisted in a plain mode.
Merge pull request #291 from elcodi/feature/dimensionable-interface

Dimensions in some entities