Declare tun factory and return its instance in new_tun_factory method

OpenVPN Adapter/TUNFactory.h

@@ -0,0 +1,18 @@
+//
+//  TUNFactory.h
+//  OpenVPN Adapter
+//
+//  Created by Sergey Abramchuk on 15.04.17.
+//
+//
+
+#import <openvpn/tun/client/tunbase.hpp>
+
+using namespace openvpn;
+
+class TUNFactory: public TunClientFactory {
+public:
+    virtual TunClient::Ptr new_tun_client_obj(openvpn_io::io_context& io_context,
+                                              TunClientParent& parent,
+                                              TransportClient* transcli) override;
+};