[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
Problem with adjustments is that we don't know too much info about them, so we would need to hack around here to guess those informations, additionally remember about Promotions order, if gift card has higher priority on list it will be called __before__ fixed discount...

I guess that we would need to extend adjustments to hold extra data for easier managing them here and in other cases.