Multi-currency support: Money + Currency class improvements (#553)

* Money improvements

* Replace all old money usage

app/models/concerns/monetizable.rb

@@ -0,0 +1,14 @@
+module Monetizable
+    extend ActiveSupport::Concern
+
+    class_methods do
+      def monetize(*fields)
+        fields.each do |field|
+          define_method("#{field}_money") do
+            value = self.send(field)
+            value.nil? ? nil : Money.new(value, currency)
+          end
+        end
+      end
+    end
+end