Enabling Future Payments

- Added feature to create future payments
- Updated Authentication model to retrieve long lived refresh token
- Updated Documentation
- Updated Sample

lib/PayPal/Api/FuturePayment.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace PayPal\Api;
+
+use PayPal\Common\PPModel;
+use PayPal\Rest\ApiContext;
+use PayPal\Transport\PPRestCall;
+
+/**
+ * Class FuturePayment
+ *
+ * @package PayPal\Api
+ */
+class FuturePayment extends Payment
+{
+
+    /**
+     * Extends the Payment object to create future payments
+     *
+     * @param null $apiContext
+     * @param      $correlationId
+     * @return $this
+     */
+    public function create($apiContext = null, $correlationId = null) {
+        if ($apiContext == null) {
+            $apiContext = new ApiContext(self::$credential);
+        }
+        if (($correlationId == null || trim($correlationId) == "")) {
+            throw new \InvalidArgumentException("correlationId cannot be null or empty");
+        }
+        $payLoad = $this->toJSON();
+        $call = new PPRestCall($apiContext);
+        $json = $call->execute(
+            array('PayPal\Rest\RestHandler'),
+            "/v1/payments/payment",
+            "POST",
+            $payLoad,
+            [
+                'Paypal-Application-Correlation-Id' => $correlationId,
+                'PAYPAL-CLIENT-METADATA-ID' => $correlationId
+            ]
+        );
+        $this->fromJson($json);
+
+        return $this;
+
+    }
+}

lib/PayPal/Api/Payment.php

@@ -26,7 +26,7 @@ class Payment extends PPModel implements IResource
     /**
      * @var
      */
-    private static $credential;
+    protected static $credential;
 
     /**
      * Set Credential
@@ -467,14 +467,14 @@ class Payment extends PPModel implements IResource
         $payLoad = "";
 
         $allowedParams = array(
-            'count'       => 1,
-            'start_id'    => 1,
+            'count' => 1,
+            'start_id' => 1,
             'start_index' => 1,
-            'start_time'  => 1,
-            'end_time'    => 1,
-            'payee_id'    => 1,
-            'sort_by'     => 1,
-            'sort_order'  => 1,
+            'start_time' => 1,
+            'end_time' => 1,
+            'payee_id' => 1,
+            'sort_by' => 1,
+            'sort_order' => 1,
         );
 
         if ($apiContext == null) {