Enabled Payment Experience

- Updated Api to enabled Payment Experience
- Updated Tests and Samples
- Added Json Validator
- Ability for PPModel to return array of self objects

lib/PayPal/Validation/JsonValidator.php

@@ -0,0 +1,28 @@
+<?php
+
+namespace PayPal\Validation;
+
+/**
+ * Class JsonValidator
+ *
+ * @package PayPal\Validation
+ */
+class JsonValidator
+{
+
+    /**
+     * Helper method for validating if string provided is a valid json.
+     *
+     * @param string $string String representation of Json object
+     * @return bool
+     */
+    public static function validate($string)
+    {
+        json_decode($string);
+        if (json_last_error() != JSON_ERROR_NONE) {
+            //Throw an Exception for string or array
+            throw new \InvalidArgumentException("Invalid JSON String");
+        }
+        return true;
+    }
+}