[Promotion] Basic gift card promotion support

Three 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)
![screenshot](https://cloud.githubusercontent.com/assets/67402/4233963/d57524e2-39b1-11e4-9102-c0351ab5c534.png)

TODO:
- [x] add migration script,
- [x] add section in user account about used gift cards/coupons(?),
- [x] extend fixtures
When using multiple promotion, it seem to deduct more than it should deduct. Eg: If I use $5 fixed discount and $200 gift card on a order of $100, it deducts $100 from the gift card where it should deduct $95.