WIP: Variant picker

How do I add an action to the stock details form that appears? Must I create a view and then somehow bind a variant to that view? How do I do that?
Send show stock details
Remove variant property from LineItemsShowController, as this is now obtainable via a straight @get('variant')
Correctly implement line item destruction
Add BaseModel

This can be used to setup associations on models, and not much else. Each model must define its own associate function so that this code knows how to associate the two objects together
Correct code for Order#advance
Finish implementing stock picker
Build URL for Shipment#adjust_items more sensibly

Spree.pathFor will use the correct path (i.e. relative to wherever spree is mounted)