Implement coupons

[torandi]

At NitroXy 12 we would like to have a coupon system. Proposed functionality:

• Create a coupon, get a code (preferably as a barcode)
• The customer use the coupon to shop. The coupon's value is reduced with the used amount (but the same code is still valid for the coupon).

[edruid]

I won't have time to implement this until nx12.

[torandi]

I could do it (or try to, don't know if I will have time either)

[edruid]

gogogo

[edruid]

So now you are a colaborator so you can actually make changes to this repo. =)