5.6.0

• The ItemDefinition gained a new key-word argument item_type that must
  be set to 'contribution' or 'donation' when creating purchase URLs for
  contributions or donations.

• Two new constants were added to the constants module:
  ITEM_TYPE_CONTRIBUTION and ITEM_TYPE_DONATION to be used in conjuction
  with the new item_type attribute on an ItemDefinition.