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)


Test code:
```
###########Test################
import numpy as np
import pandas as pd

# For normal numpy arrays
itemscores = [[1,2,3,5,4,1],
              [3,6,5,6,4,1]]

print "Cronbach alpha = ", cronbach_alpha(itemscores)

# For arrays with Nan values (they should be dropped)
itemscoresNAN = [[1,2,3,5,4,1],
              [3,6,5,6,4,1],
              [3,6,5,6,np.nan,1]]

print "Cronbach alpha Nan = ", cronbach_alpha(itemscoresNAN)

#Pandas dataframe

df = pd.DataFrame({'TFD' : ['A', 'b', 'C','D','E','F'],
                    'Data1' : [1,2,3,5,4,1],
                    'Data2' : [3,6,5,6,4,1]}).set_index('TFD')
df=df.transpose()
print "Cronbach alpha (pd) = ", cronbach_alpha(df)
```