[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.
Great point, yeah.  This comment ought to be updated to say something like that.  It's probably worth making it clear though (maybe also in a README note) since both we and @tesserakt have looked into doing the "partial on-hand/warehouse pick-up" approach even in current Spree, which will result in incorrect tax calculations.

And just to be sure we're on the same page -- `origincodeline` is still incorrect, though I don't know how often that actually makes a difference in tax calculations.  I assume Avatax asks for it because it can make a difference but I don't know.