Improvements to invoice display and other misc items

modules/invoice/classes/Invoicable.php

@@ -0,0 +1,16 @@
+<?php defined('SYSPATH') or die('No direct access allowed.');
+
+/**
+ * This interface ensures that all items have the required invoice methods
+ *
+ * @package    Invoice
+ * @category   Interface
+ * @author     Deon George
+ * @copyright  (c) 2009-2013 Open Source Billing
+ * @license    http://dev.osbill.net/license.html
+ */
+interface Invoicable {
+	// Render an invoice line item.
+	public function invoice_item($item_type);
+}
+?>