add graphviz plot for decision trees

- added methods to write decision tree into file in dot format
- added python_modular graphical example for the same

The pdf output of the python graphical example can be found [here](https://www.dropbox.com/s/qxby3349yxw3mmi/decision_tree.pdf). The dot file created can be found [here](https://gist.github.com/mazumdarparijat/11373739)