add bounding method in factor graph model for submodularity

- added bounding method for solving submodularity issue in graph cuts (currently only PrimalMosekSOSVM supports this feature)
- updated multilabel classification example
- solved hinge loss < 0 issue in SOSVMHelper
OK. So maybe add an example for GraphCut, so that people will know how to use GraphCut. A python example is fine.