make python callbacks work

this diff should make calling python from C++ in ROOT work in this example.

test:
    python myclass.py
output:
RooFit v3.60 -- Developed by Wouter Verkerke and David Kirkby 
                Copyright (C) 2000-2013 NIKHEF, University of California & Stanford University
                All rights reserved, please read http://roofit.sourceforge.net/license.txt

Hi from C++!
Hi from python!