added MPLP

Max-product LP relaxation inference for factor graph model.
Unit-test has been added.
Examples will be added soon.
Regions actually play the same role as factors, which could be viewed as a generalization of factors. Indeed, in the following paper, region is defined as a set of factors. Anyway, I think the explanation here is not very precise. BTW, The dual variables in dual LPR are messages. 

Jonathan S. Yedidia et al. Constructing Free Energy Approximations and Generalized Belief Propagation Algorithms. 2004.