Updated Billing Plan to have Delete Plan sample code.

- Added helper method in Plan.php to allow deleting a billing plan
- Fixes #171

sample/billing/DeletePlan.php

@@ -0,0 +1,24 @@
+<?php
+
+// # Delete Plan Sample
+//
+// This sample code demonstrate how you can delete a billing plan, as documented here at:
+// https://developer.paypal.com/webapps/developer/docs/api/#retrieve-a-plan
+// API used: /v1/payments/billing-plans
+
+// Retrieving the Plan object from Create Plan Sample
+/** @var Plan $createdPlan */
+$createdPlan = require 'CreatePlan.php';
+
+use PayPal\Api\Plan;
+
+try {
+    $result = $createdPlan->delete($apiContext);
+} catch (Exception $ex) {
+    ResultPrinter::printError("Deleted a Plan", "Plan", $plan->getId(), null, $ex);
+    exit(1);
+}
+
+ResultPrinter::printResult("Deleted a Plan", "Plan", $plan->getId(), null, null);
+
+return $plan;