ENH: PHReg formula improvements 

This PR allows extra arguments of array type to be passed to the PHReg from_formula method as variable names.  The data are read out of the same data object that is used for the formula.
Since patsy 1.2 (IIRC) we have missing='drop' as default in from_formula because we don't prevent patsy from dropping missing rows.
We still have the open issues about conflicts between patsy's and our missing value handling.