Bring Spree::Stock::Quantifier in to OFN

This is the original unmodified Class from Spree. Modifications added in subsequent commits.

app/models/spree/stock/quantifier.rb

@@ -0,0 +1,28 @@
+module Spree
+  module Stock
+    class Quantifier
+      attr_reader :stock_items
+
+      def initialize(variant)
+        @variant = variant
+        @stock_items = Spree::StockItem.joins(:stock_location).where(:variant_id => @variant, Spree::StockLocation.table_name =>{ :active => true})
+      end
+
+      def total_on_hand
+        if Spree::Config.track_inventory_levels
+          stock_items.sum(&:count_on_hand)
+        else
+          Float::INFINITY
+        end
+      end
+
+      def backorderable?
+        stock_items.any?(&:backorderable)
+      end
+
+      def can_supply?(required)
+        total_on_hand >= required || backorderable?
+      end
+    end
+  end
+end