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
Alright! Tag a TODO there, but change assert to REQUIRE, and say something like temporally GraphCut doesn't support edge features.