BACKWARDS INCOMPATIBLE: get_items_and_qantities must return 3-tuple

* Before this change we were only saving the quantity of a purchased item and
  it's transaction. This is not sufficient for transactions that have more than
  one item. From now on we will save quantity and price at the time of purchase
  and optinally a GFK to anything so that we can remember what this item was
  about. We also have the new field 'identifier' which allows us to calculate
  sums for certain types of items for a transaction (i.e. only shipping costs).
* For your current shop you must make sure that your implementation of
  get_items_and_quantities now reuturns a list of 3-tuples:
  [(item, quantity, content_object), ...]
* You can probably easily achieve that by just returning ``None`` for the
  content object and everything shoudl still work as before.
Released v1.0