lock down aws-sdk version due to https://github.com/aws/aws-sdk-ruby/issues/273

Fixes #3924
fix comment to refer to checkout_step?

respond_to? isn't used on master any more and refers to older code

Fixes #3923
Use correct namespace for 'payment_method_not_supported' translation in payment/processing.rb

Fixes #3935
Fixes #3934
Add missing 'payment_method_not_supported' translation
Fix whitespace
Change input padding to allow more horizontal room for number fields

* The 10px horizontal padding causes number fields, like the
  quantity field, to not fully display the number in the space
  allowed.

Fixes #3938
Bump Ransack to 1.1.0
Generate unique indexes for new styles attachments form

Fixes #3940
move gemspec in spree core forward
Spree.user_class class variable accepts a symbol value

Fixes #3930
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
Merge branch '2-0-stable' of https://github.com/spree/spree into 2-0-stable

* '2-0-stable' of https://github.com/spree/spree:
  Timestamps to json give us miliseconds precision
  Spree.user_class class variable accepts a symbol value
  move gemspec in spree core forward
  Generate unique indexes for new styles attachments form
  Bump Ransack to 1.1.0
  Change input padding to allow more horizontal room for number fields
  Fix whitespace
  Add missing 'payment_method_not_supported' translation
  Use correct namespace for 'payment_method_not_supported' translation in payment/processing.rb
  fix comment to refer to checkout_step?
  lock down aws-sdk version due to https://github.com/aws/aws-sdk-ruby/issues/273
Only validate the month/year for CC info if there is no payment profile
A payment profile can either be a customer id or a payment profile id
Only show last digits on payment form if they exist
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
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

Conflicts:
	api/spec/controllers/spree/api/orders_controller_spec.rb
Add sku to line item attr_accessible

Allow users to find variant by sku when importing orders

Conflicts:
	api/app/models/spree/line_item_decorator.rb
Add `ship_total` to api order_attributes
Make OrderInventory aware of `track_inventory_levels`

It should not create backordered inventory units when it is set `false`