Set shipping net amount to zero when saving orders

The shipping amount in s_order is set to zero since these informations are stored within the basket of the corresponding order. In terms of consistency and to avoid confusion the shipping net amount should also be set to zero.
Fix calculation of shipping costs

Currently the shipping tax is hardcoded to 19 %, regardless of the configured shipping tax for the current dispatch. This commit changes this behaviour by calculating the shipping tax using the shipping amount and shipping net amount from the order.
Merge pull request #4 from VIISON/fix_shipping_tax

Fix calculation of shipping costs
Merge pull request #2 from VIISON/fix_shipping_details

Set shipping net amount to zero when saving orders