Shifu 0.2.6 Add Skewness and Kurtosis Stats

Two other statistical measures we can consider implementing for future releases are skewness (3rd central moment) and kurtosis (4th central moment) to assess heaviness of tails and skew in data. They should be pretty straightforward to implement directly leveraging standard deviation (2nd central moment). Formulae are very similar:

http://www.itl.nist.gov/div898/handbook/eda/section3/eda35b.htm

Such two values are generatred in ColumnConfig.json.