Adds customer helper to CustomerGroup resource

Fixes #39

lib/shopify_api/resources/customer_group.rb

@@ -1,4 +1,9 @@
 module ShopifyAPI
   class CustomerGroup < Base
+
+    def customers
+      Customers.find(:all, :params => {:customer_group_id => self.id})
+    end
+    
   end
 end