Change Field.clone() to preserve .parent

The reasons for this (and some alternatives) are listed at
Pylons#187 (comment)

deform/field.py

@@ -278,12 +278,15 @@ def clone(self):
         attribute of the node is not cloned; instead the field
         receives a new order attribute; it will be a number larger
         than the last rendered field of this set.  The parent of the cloned
-        node will become ``None`` unconditionally."""
+        node will be retained by the clone."""
         cloned = self.__class__(self.schema)
         cloned.__dict__.update(self.__dict__)
         cloned.order = next(cloned.counter)
         cloned.oid = 'deformField%s' % cloned.order
-        cloned._parent = None
+        parent = self.parent
+        if parent is not None:
+            parent = weakref.ref(parent)
+        cloned._parent = parent
         children = []
         for field in self.children:
             cloned_child = field.clone()