Updated Invoicing APIs

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

sample/invoice/GetNextInvoiceNumber.php

@@ -0,0 +1,24 @@
+<?php
+
+// # Get Next Invoice Number Sample
+// This sample code demonstrate how you can retrieve
+// the next invoice number.
+
+require '../bootstrap.php';
+use PayPal\Api\Invoice;
+
+// ### Get Next Invoice Number
+// To generate the successive invoice number for the merchant, use below code.
+// (See bootstrap.php for more on `ApiContext`)
+try {
+    $number = Invoice::generateNumber($apiContext);
+} catch (Exception $ex) {
+    // NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+ 	ResultPrinter::printError("Get Next Invoice Number", "InvoiceNumber", null, null, $ex);
+    exit(1);
+}
+
+// NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+ ResultPrinter::printResult("Get Next Invoice Number", "InvoiceNumber", null, $number, $number);
+
+return $number;