support for primary keys fields other than id

I've seen pk is used all over the models.py file except in get_product. This generates errors if the product has not an id field (but another primary key). The proposed patch solves the problem. 
Merge pull request #6 from furins/patch-2

support for primary keys fields other than id

I'm merging this in without tests on account of the fact that it's standard practice to use pk instead of id.

// cc @bmentges I'll bump the version number, can you push to the cheeseshop?