added 'add_tun' method in node.py

src/netns/node.py

@@ -116,6 +116,12 @@ class Node(object):
             setattr(i, k, v)
         return i
 
+    def add_tun(self, **kwargs):
+        i = netns.interface.TunNodeInterface(self)
+        for k, v in kwargs.items():
+            setattr(i, k, v)
+        return i
+
     def import_if(self, interface):
         return netns.interface.ImportedNodeInterface(self, interface)