Removing Dependency from SDK Core Project

- Copied files required for Rest API SDK
- Removed PPApiContext and directly connected APIContext with PPConfigManager
- Removed duplicate data storage of configuration and credentials.
- Code Style Fixes
- Remove build.xml file as it is not required anymore
- Updated the samples
- Updated the documentations

lib/PayPal/Core/PPConstants.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace PayPal\Core;
+
+/**
+ * Class PPConstants
+ * Placeholder for Paypal Constants
+ *
+ * @package PayPal\Core
+ */
+class PPConstants
+{
+
+    const SDK_NAME = 'rest-api-sdk-php';
+    const SDK_VERSION = '0.11.0';
+
+    const REST_SANDBOX_ENDPOINT = "https://api.sandbox.paypal.com/";
+    const REST_LIVE_ENDPOINT = "https://api.paypal.com/";
+}