support for different margins (per ordergroup)

Some foodcoops would like to give members the option to participate without volunteering and paying a hight price. This feature, when enabled in the foodcoop configuration, allows an admin to select a margin from a preconfigured list. The `fc_price`s for that ordergroup are then different.

![foodcoop_margin_edit](https://cloud.githubusercontent.com/assets/503804/2999627/36006084-dd18-11e3-9a8d-5d9877bc04e4.png)