Allow shipping to multiple stock locations

[seand7565]

Right now the gem assumes the first stock location it finds is the only one - you can't adjust the address in the "ship to" field on purchase orders.

To start out with, we could add a stock_location selector to the PO and use whichever location is selected as the ship to address.

In the future, it might be nice for the system to track stock needs by stock location, rather than stock movements across the entire store. For example, instead of creating a PO for 100 widgets because we need 40 in location A and 60 in location B, create two POs - one for 40 with a ship to address of location A and another for 60 shipping to location B.

Our store does not have multiple stock locations, so any input from those who do would be greatly appreciated!