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
Fix invalid erb comment syntax in admin/taxons/edit.html.erb

Fixes #3926
Update _delivery.html.erb to enforce space between shipping method name and price

My `1-3-stable` site was showing the Shipping Method name and price running together - for some reason the space here wasn't registering. Converting it to a non-breaking space seemed to work, and so just to be certain about the desired behavior, I'd propose making it `&nbsp;` in Spree itself, in case there are other people encountering the same issue.

Fixes #3941
Ensure callbacks are not called when nullifying adjustment originators

Fixes #3964
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