added MPLP

Max-product LP relaxation inference for factor graph model.
Unit-test has been added.
Examples will be added soon.
Yes @hushell, that was pretty much my point. I agree with you, it is a good idea to do this in another PR. Let me know if you want this one merged and then merge MDArray and SGNDArray, or the other way around. @Jiaolong, any preference from your side?