Updated Invoicing APIs

- Updated Model objects.
- Updated Samples.
- Updated Tests.

lib/PayPal/Api/InvoiceNumber.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace PayPal\Api;
+
+use PayPal\Common\PayPalModel;
+
+/**
+ * Class InvoiceNumber
+ *
+ * The next invoice number
+ *
+ * @package PayPal\Api
+ *
+ * @property string number
+ */
+class InvoiceNumber extends PayPalModel
+{
+    /**
+     * The next invoice number.
+     *
+     * @param string $number
+     *
+     * @return $this
+     */
+    public function setNumber($number) {
+        $this->number = $number;
+        return $this;
+    }
+
+    /**
+     * The next invoice number.
+     *
+     * @return string
+     */
+    public function getNumber() {
+        return $this->number;
+    }
+}