More Vault API Samples

- Updated Samples for Vault APIs
- Updated Payment Execute to be more verbose
- Delete Plan Sample Update
- Fixes #168

sample/billing/ExecuteAgreement.php

@@ -1,14 +1,19 @@
 <?php
 
+// #Execute Agreement
+// This is the second part of CreateAgreement Sample.
+// Use this call to execute an agreement after the buyer approves it
 require __DIR__ . '/../bootstrap.php';
-session_start();
+
+// ## Approval Status
+// Determine if the user accepted or denied the request
 if (isset($_GET['success']) && $_GET['success'] == 'true') {
 
     $token = $_GET['token'];
-
     $agreement = new \PayPal\Api\Agreement();
-
     try {
+        // ## Execute Agreement
+        // Execute the agreement by passing in the token
         $agreement->execute($token, $apiContext);
     } catch (Exception $ex) {
         ResultPrinter::printError("Executed an Agreement", "Agreement", $agreement->getId(), $_GET['token'], $ex);