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/Exception/PPInvalidCredentialException.php

@@ -0,0 +1,36 @@
+<?php
+
+namespace PayPal\Exception;
+
+/**
+ * Class PPInvalidCredentialException
+ *
+ * @package PayPal\Exception
+ */
+class PPInvalidCredentialException extends \Exception
+{
+
+    /**
+     * Default Constructor
+     *
+     * @param string|null $message
+     * @param int  $code
+     */
+    public function __construct($message = null, $code = 0)
+    {
+        parent::__construct($message, $code);
+    }
+
+    /**
+     * prints error message
+     *
+     * @return string
+     */
+    public function errorMessage()
+    {
+        $errorMsg = 'Error on line ' . $this->getLine() . ' in ' . $this->getFile()
+            . ': <b>' . $this->getMessage() . '</b>';
+        return $errorMsg;
+    }
+
+}