Reorganize some Redis commands classes.

lib/Predis/Commands/StringSetMultiplePreserve.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace Predis\Commands;
+
+class StringSetMultiplePreserve extends StringSetMultiple {
+    public function getId() {
+        return 'MSETNX';
+    }
+
+    public function parseResponse($data) {
+        return (bool) $data;
+    }
+}