resolve forecast incompatibilities in statsmodels
cross-linking to statsmodel issue

https://github.com/statsmodels/statsmodels/issues/168

We may have to change some of the forecast dates code when pandas switches over to the ordinal timestamp representation. Right now, our current implementation is broken with the new datetime branch installed. To replicate, install pandas datetime branch and run examples/tsa/ex_dates.py. Make sure to write tests for this whenever the pandas code settles down.
