Renaming Namespaces and Organizing Classes

- Updated OpenId classes to be in API namespace
- Updated PP Naming Convention to PayPal Naming Convention
- FormatConverter Class got its own namespace
- Handlers are grouped in Handler namespace
- Samples and Tests Updated Accordingly

lib/PayPal/Core/PayPalConstants.php

@@ -0,0 +1,22 @@
+<?php
+
+namespace PayPal\Core;
+
+/**
+ * Class PayPalConstants
+ * Placeholder for Paypal Constants
+ *
+ * @package PayPal\Core
+ */
+class PayPalConstants
+{
+
+    const SDK_NAME = 'PayPal-PHP-SDK';
+    const SDK_VERSION = '0.16.1';
+
+    const REST_SANDBOX_ENDPOINT = "https://api.sandbox.paypal.com/";
+    const OPENID_REDIRECT_SANDBOX_URL = "https://www.sandbox.paypal.com/webapps/auth/protocol/openidconnect";
+
+    const REST_LIVE_ENDPOINT = "https://api.paypal.com/";
+    const OPENID_REDIRECT_LIVE_URL = "https://www.paypal.com/webapps/auth/protocol/openidconnect";
+}