Adds a Prepaid & Charge billing mode to carrier.account.mixin and
overrides the stock.picking method that sets the shipping cost on sales
orders after shipping. This modifies the behaviour of "invoicing policy"
on delivery.carrier when a delivery billing mode is selected on the SO
or picking.
When adding transport charges to a sale order via the "Add Shipping"
button, the line's description now contains the delivery billing mode
and account number when applicable.
Fixes#112
