[PoC][Promotion] Basic gift card promotion support

Two screenshots:
![screenshot](https://cloud.githubusercontent.com/assets/67402/4232259/8bc80b00-399a-11e4-82dd-6125c66038f7.png)
![screenshot](https://cloud.githubusercontent.com/assets/67402/4232260/8be369d6-399a-11e4-92ad-814dc36d3298.png)

TODO:
- [ ] add migration script,
- [ ] add section in user account about used gift cards/coupons(?)