- If it omits setting the counters, which are notifyoutv4, notifyoutv6,
   xfrrej, and xfrreqdone, they are set to None as defaults when the object is
   initiating. Then when each counter is invoked in some method, it checks
   whether the counter is Nonetype or not. Unless the counter is NoneType, it
   invokes the counter.  After that unless the counter is callable, a TypeError
   exception would be raised.

 - added some tests for this changes

 - removed some dead code from xfrout.py.in