Add Product#product_distribution_for
Create line item adjustments for product distributions
Weight calculator will calculate against a single line item as well as an order
Create adjustments so computation is performed against line item. Simplify specs now that weight calculator is compatible with line items.
Clean up old adjustments when changing distributor
Adjustments on LineItems don't modify the order total. Make adjustments on Order instead.
Upgrade Rails to 3.2.14, use forked spree with calculators that are compatible with LineItems
When updating product distribution charge, skip line items that don't have a product distribution