Add more infos to the invoice page

When a foodcoop does not use the balancing feature the invoice page is
the main page for informations. Add all related information to it, so
people do not need to open the balancing page.

app/models/delivery.rb

@@ -20,7 +20,24 @@ class Delivery < ActiveRecord::Base
   def includes_article?(article)
     self.stock_changes.map{|stock_change| stock_change.stock_article.id}.include? article.id
   end
-  
+
+  def sum(type = :gross)
+    total = 0
+    for sc in stock_changes
+      article = sc.stock_article
+      quantity = sc.quantity
+      case type
+        when :net
+          total += quantity * article.price
+        when :gross
+          total += quantity * article.gross_price
+        when :fc
+          total += quantity * article.fc_price
+      end
+    end
+    total
+  end
+
   protected
   
   def stock_articles_must_be_unique