Rc2.1

Allmost only python changes,but a tiny change in MQpbo.

-visitor has been used with visitor.visit  instead of visit() 
- and a default value for Parameter in Constructor has Been added


Python Changes :
- added mqpbo to python
- added partialOptimality interface forn qpbo and mqpbo
- added 'integrator' as accumulator for bp and trwbp 
- added interface to get marginals and factor marginals for bp and trwbp