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/Handler/IPPHandler.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace PayPal\Handler;
+
+/**
+ * Interface IPPHandler
+ *
+ * @package PayPal\Handler
+ */
+interface IPPHandler
+{
+    /**
+     *
+     * @param \Paypal\Core\PPHttpConfig $httpConfig
+     * @param string $request
+     * @param mixed $options
+     * @return mixed
+     */
+    public function handle($httpConfig, $request, $options);
+}