Declare tun factory and return its instance in new_tun_factory method

OpenVPN Adapter/TUNFactory.mm

@@ -0,0 +1,18 @@
+//
+//  TUNFactory.m
+//  OpenVPN Adapter
+//
+//  Created by Sergey Abramchuk on 15.04.17.
+//
+//
+
+#import "TUNFactory.h"
+
+// !!! tuncli.hpp may be used as example of implementation
+
+TunClient::Ptr TUNFactory::new_tun_client_obj(openvpn_io::io_context& io_context,
+                                              TunClientParent& parent,
+                                              TransportClient* transcli)
+{
+    return nullptr;
+}