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/Exception/PayPalMissingCredentialException.php

@@ -0,0 +1,37 @@
+<?php
+
+namespace PayPal\Exception;
+
+/**
+ * Class PayPalMissingCredentialException
+ *
+ * @package PayPal\Exception
+ */
+class PayPalMissingCredentialException extends \Exception
+{
+
+    /**
+     * Default Constructor
+     *
+     * @param string $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;
+    }
+
+}