[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
I guess this can be added later if needed, now it's only one case and TBH I don't see many other use cases for such "configuration"...