[WIP]
introduce the auto_push concern
Closes #32

Conflicts:
	app/models/spree/shipment_decorator.rb
	app/models/spree/stock_item_decorator.rb
	app/models/spree/variant_decorator.rb
	app/serializers/spree/hub/stock_item_serializer.rb
	spec/models/spree/stock_item_spec.rb
	spec/serializers/spree/hub/order_serializer_spec.rb
	spec/serializers/spree/hub/shipment_serializer_spec.rb
	spec/serializers/spree/hub/stock_item_serializer_spec.rb
	spec/serializers/spree/hub/variant_serializer_spec.rb
	spec/spec_helper.rb