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
@hushell If edge features are available, I don't think we can set any constrains on w to ensure submodularity. :(