handled special case in anova_f_test, CHAID notebook added

- handled a special case in anova f test which was causing error in boston housing dataset
- CHAID section added in notebook

[View](https://gist.github.com/mazumdarparijat/985a516614cb358b8fb7)
So 89.5% accuracy. [I got 97% last summer with LMNN](http://nbviewer.ipython.org/gist/iglesias/6576096), in case you might find it a challenge to improve the accuracy with CHAID.