cross-validation enabled for TreeMachine, added CART notebook

- cross validation enabled by adding empty store_model_parameters method
- CART notebook added

[CART notebook output](http://nbviewer.ipython.org/gist/mazumdarparijat/326c688ac71c80ff839d)
If I understand this correctly, the mean squared error obtained is not a relative error, but an absolute error. Then, only the 0.5 value does not tell much about the actual error made. That is, it depends on the range of values taken by the ground truth whether 0.5 is a large error or not. It would be nice if you could write a number to give an idea of these range of values, or use a relative error instead.

It might be that I am understanding something wrong though!