Adding Cronbach Alpha (Output format needs to be checked)

Added Cronbach Alpha function with improve code from

https://github.com/statsmodels/statsmodels/issues/1272

Still may need a function to omit arrays with NaN.

Don't know how you guys want the return options but someone might want to adjust the return values since R yields something like:

Cronbach's Alpha = calpha
Sample size = nitems
Number of items = len(tscores)