add ability to read Stata files

[jseabold]

Note to self. I think it makes sense to move or copy the Stata file handling from statsmodels to pandas rather than having a return_pandas option in statsmodels. DataFrame.from_dta would be helpful.

https://github.com/statsmodels/statsmodels/blob/master/statsmodels/iolib/foreign.py

[jreback]

closed by #3270