Timestamps to json give us miliseconds precision

Motivation here was ensure api/orders return updated_at with miliseconds
couldn't find any other easy way to do so. Rails 4 already does it
default

Conflicts:
	core/CHANGELOG.md
	core/lib/spree/core.rb
Introduce channel attr to Order model

Could be set when importing orders from other storefronts

Conflicts:
	api/CHANGELOG.md
	api/spec/controllers/spree/api/orders_controller_spec.rb
	core/CHANGELOG.md

Conflicts:
	api/CHANGELOG.md
	api/app/controllers/spree/api/orders_controller.rb
	api/app/helpers/spree/api/api_helpers.rb
	api/spec/controllers/spree/api/orders_controller_spec.rb
	core/CHANGELOG.md

Conflicts:
	api/CHANGELOG.md
	api/app/helpers/spree/api/api_helpers.rb
	api/spec/controllers/spree/api/orders_controller_spec.rb
	core/CHANGELOG.md
Better error messages when importing orders (api)
Set state_name when state record not found [api]
Scope state per country on api import script
Make line_items attr handle more flexible on api

When importing orders or creating orders and line items at once the
line_items key on order payload would provide an array not a hash. The
API need to consider then that both hash and array might be provided and
handle it gracefully
Add sku to line item attr_accessible

Allow users to find variant by sku when importing orders
Ensure item quantities are converted to integer

Prevents `String can't be coerced into Fixnum` when importing orders.
To increase a quantity for existing line item we need to pass a integer
always not a string
Fix state_name spec for order import script
Query only valid payments for order confirmation_required?

Prevent a confirmation step in cases where a previous selected payment
method which required confirmation failed and customer picked one that
doesnt require confirmation next

Fixes #3970

Conflicts:
	core/app/models/spree/order.rb
	core/spec/controllers/spree/checkout_controller_spec.rb
Fix payments_spec set up
restart the checkout_flow when modifying the order.

Fixes #3990
Fixes #4027