Updated CronbachAlpha from pull #1273

Updated code from 
https://github.com/statsmodels/statsmodels/pull/1273

Fixes:
*Fixed code for pep-8 style and recommendations from dengemann and josef-pkt (I think)
*Added setting to take out np.Nan arrays

Still needed: 
Summary (I don't know what people want)
Cronbach's Alpha = calpha
Sample size = nitems
Number of items = len(tscores)