Enabled Billing Plans and Agreements APIs

- Added API Classes, Samples, and Tests
- Updated Functional Tests
- Updated Documentation with new SDK Name
- Updated Few Samples to use newer nicer result page

sample/billing/ExecuteAgreement.php

@@ -0,0 +1,22 @@
+<?php
+
+require __DIR__ . '/../bootstrap.php';
+session_start();
+if (isset($_GET['success']) && $_GET['success'] == 'true') {
+
+    $token = $_GET['token'];
+
+    $agreement = new \PayPal\Api\Agreement();
+
+    try {
+        $agreement->execute($token, $apiContext);
+    } catch (PayPal\Exception\PPConnectionException $ex) {
+        echo "Exception: " . $ex->getMessage() . PHP_EOL;
+        var_dump($ex->getData());
+        exit(1);
+    }
+    ResultPrinter::printResult("Executed an Agreement", "Agreement", $agreement->getId(), $_GET['token'], $agreement);
+
+} else {
+    ResultPrinter::printResult("User Cancelled the Approval", null);
+}