[DO NOT MERGE] Rough draft for cleaning up addresses logic

This is just a quick pass at tentative code relating to https://github.com/bonobos/spree_avatax/issues/34.  I haven't tested this out at all or written specs.  It's just for discussion.
@jordan-brough that's exactly the situation we are in, a split order where one part of the order is a warehouse pickup and needs tax.