Order API Support

- Added Order API Support
- Updated Sample Code to demonstrate
- Updated Sample Docs
- Fixes #237

sample/payments/OrderCapture.php

@@ -0,0 +1,51 @@
+<?php
+// #Capture Order Sample
+// To authorize an order payment, pass the orderId in the URI of a POST call. This begins the process of confirming that funds are available until it is time to complete the payment transaction.
+// API used: POST /v1/payments/orders/<Order-Id>/authorize
+
+/** @var \PayPal\Api\Payment $payment */
+$payment = require __DIR__ . '/ExecutePayment.php';
+
+use PayPal\Api\Capture;
+use PayPal\Api\Amount;
+
+// ### Approval Status
+// Determine if the user approved the payment or not
+if (isset($_GET['success']) && $_GET['success'] == 'true') {
+
+    // ### Retrieve the order
+    // OrderId could be retrieved by parsing the object inside related_resources.
+    $transactions = $payment->getTransactions();
+    $transaction = $transactions[0];
+    $relatedResources = $transaction->getRelatedResources();
+    $relatedResource = $relatedResources[0];
+    $order = $relatedResource->getOrder();
+
+    // ### Create Capture Object
+    // with Amount in it
+    $capture = new Capture();
+    $capture->setIsFinalCapture(true);
+    $capture->setAmount(new Amount(
+        '{
+            "total": "2.00",
+            "currency": "USD"
+        }'
+    ));
+
+    try {
+        // ### Capture Order
+        // Capture the order by passing capture object we created.
+        // We will get a new capture object back.
+        $result = $order->capture($capture, $apiContext);
+        ResultPrinter::printResult("Captured Order", "Capture", $result->getId(), $capture, $result);
+    } catch (Exception $ex) {
+        ResultPrinter::printError("Captured Order", "Capture", null, $capture, $ex);
+        exit(1);
+    }
+
+    return $result;
+
+} else {
+    ResultPrinter::printResult("User Cancelled the Approval", null);
+    exit;
+}