Updated Invoicing APIs

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

sample/index.php

@@ -1186,6 +1186,17 @@ if (PHP_SAPI == 'cli') {
                 </div>
                 <!-- List group -->
                 <ul class="list-group">
+                    <li class="list-group-item">
+                        <div class="row">
+                            <div class="col-md-8"><h5>Generate Next Invoice Number</h5></div>
+                            <div class="col-md-4">
+                                <a href="invoice/GetNextInvoiceNumber.php" class="btn btn-primary pull-left execute"> Try It <i
+                                        class="fa fa-play-circle-o"></i></a>
+                                <a href="doc/invoice/GetNextInvoiceNumber.html" class="btn btn-default pull-right">Source <i
+                                        class="fa fa-file-code-o"></i></a>
+                            </div>
+                        </div>
+                    </li>
                     <li class="list-group-item">
                         <div class="row">
                             <div class="col-md-8"><h5>Create an Invoice</h5></div>