[POC] Add initial CustomizationBundle

Hi all,

This is a first implementation of #363.

The new bundle offers customization: it means you have several attributes (let's say tee-shirt printing) that you can attach to Subject (in our case, the tee-shirt product). When attached, you then register a value (the printed text on your tee-shirt) on a SubjectInstance (in our case, the OrderItem of that tee-shirt in the cart).

For now it's all it does. Of course, we miss:
* Allow all type of values (text, date, file, etc.). This first step only provide text values.
* Calculator to handle the price of those customizations. It could be a per character price, etc.
* Your ideas.

I'm still struggling with the CartItem form of the product page: need to add CustomizationValues fields regarding the available Customizations of the product and the values that can already exist... a bit tricky, forms are not my preferred  part.

In the meanwhile, I'd like to have feedbacks on what is already done =)

Thanks