Master pos fidelity comeback fva

This branch implements a basic fidelity card system
- A Fidelity Card model contains the rules used to compute points. You can win points for
  * a fixed amount for each order
  * points for the money spent
  * points per product sold
- You can define the rounding used so that points can represent a virtual currency 
- Products can ignore the fidelity points system and are thus not counted for the rules above.
- Products can win extra points, or made to lose points. You can thus implement the rewards as
  products that ignore rules and have a negative amount of points