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
GraphCuts can be used for general s-t graph:
```
g = GraphCut(5, 6)

# add termainal-connected edges
# i.e., SOURCE->node_i and node_i->SINK
g.add_tweights(0, 4, 0)
g.add_tweights(1, 2, 0)
```
http://nbviewer.ipython.org/gist/Jiaolong/483de7d03865a373a51a