0.4.0: Use STI for purchase items instead of types

Use single-table inheritance for purchase items.
Make the shipping package compatible with openbuildings/purchases:0.5.*
Basically Model_Purchase_Item_Shipping is replacing the "shipping" type
and the Jam_Behavior_Shippable_Purchase_Item behavior.

0.3.19

Update depenancies

0.3.18

Fix tests related to Jam_Range. Updated dependencies.

0.3.17

fix regression

0.3.16

Fix regression

0.3.15

Group_Shipping_Items::parse_form_values more percise handling

0.3.14

make it more tollerable to wrong data

0.3.13

make a more user friendly delivery time format

0.3.12

make shippings able to tolerate missing shipping_location, and recrea…

0.3.11

tolerate shipping group without price