[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
@stloyd The order total `$amount` in here already include the gift card discount. This cause incorrect gift card balance.

![screen shot 2014-09-11 at 9 57 39 pm](https://cloud.githubusercontent.com/assets/193112/4234970/f018ec66-39bb-11e4-82ac-bc526eaf3309.png)

![screen shot 2014-09-11 at 9 57 49 pm](https://cloud.githubusercontent.com/assets/193112/4234975/f32d668e-39bb-11e4-9546-624e81c67e48.png)

Incorrect remaining balance. It should be $0, now it is $50 - $40.37 = $9.63