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
Last thing about this PR: how do we deal with the case where edge features are available? then w_dim can be 8 for example? when dimension of edge feature is 2.