Manage in a Generic way a cart discount

Trouble:

- No generic way to deal with discount on order and manage the related taxes !
- We can add some stuffs like additionnal shipping costs and so, but no generix way to deal with + or - on an order
- No way to deal with the "contre-remboursement" shipping method (the customer pay at his door, which generate more expense that need to be known on the shipping)

Solution :

- Provide one way to link a specific modificator on the SO price and a product (like shipping costs, discount won by loyality points)
- Find a solution (TO BE DEFINE) to apply the good taxe on it
- Add a checkbox on shipping method "is contre-remboursement" and if the packing contain this method display a new field with the price

This could be used by some carrier to know how much the need to ask when delivering the order.