Need ability to pass currency option for multi-currency.
Need to only try specifying currency in case order doesnt exist yet.
Update Rails Version in README.md

I just demo-ed Spree the other day for a talk and had to change my rails version from 4.0.4 to 4.0.5 to get everything to work (obviously incredibly minor but I think worth fixing.)
make sure we only build the year when it's there. Validtion error otherwise.

Fixes #4725
Document that master branch needs Rails 4.1

Closes #4735
Fix broken icons in the admin interface.

Conflicts:
	backend/app/assets/stylesheets/spree/backend/shared/_icons.scss

Fixes #4731
Reorder short circuit evaluation of Estimator#shipping_methods

Fixes #4730
Make sure line_item is assigned in specs

As of dcbd5de we expect line_items to always be associated with an
inventory_unit. Ensure we are assigning that in specs.
Importer::Order should connect Invntory to Items

The Spree::Core::Importer::Order allows importing inventory units via
the API.

Since InventoryUnits should now be associated with LineItems, this ties
the unit to the first line_item matching the imported variant.
Remove order association from inventory_units

There is no need to store the order_id on an inventory_unit, as they are
tied to both a line_item in an order and a shipment in an order.
Instead, we can delegate order to the shipment.

This should hopefully allow some future simplification as they are more
normalized.
Base Shipment#to_package off of Shipment#manifest

This avoids duplication and is correct in the case that the variant
being shipped is not the same as that of the line_item.
optimized Shipment#manifest
add quantity to inventory_units

For now always setting to 1
fix availability validator to consider quantity
Fix Shipment#manifest to consider quantity
simplify OrderInventory
Add inventory_unit#remove and fix OrderInventory
Update shipments.js to understand quantities
fill_backorders
set_up_inventory takes a quantity