REF: Results.predict convert to array and adjust shape

Results.predict
convert to ndarray, and check and adjust shape, with unit_tests

closes #1032
supersedes #1178

I tried to let pandas through, but it doesn't help. np.dot for example returns an ndarray.
I left the commented out code for testing DataFrame in the generic tests, but it needs special handling on before return.
I don't know if we have any wrapper code for predict. I didn't see any.


Some comments on the requirements

- CountModels require 2d even if it's only a single prediction because of exog.shape[1]
- single regressor (OLS example) needs reshape to column_vector as in AR(MA)
- I'm not sure we always have model.exog as 2d, so I check also for 1d model.exog
- I didn't find self.data.k_exog or self.model.data.k_exog, so I did a model.exog.shape check directly