Mice

This is a WIP PR. I have implemented a statsmodels class that receives a dataset with missing values and imputes the missing values using chained equations. It can then fit an analysis model to the fully imputed data. The code gives sensible results for a real dataset but has not been rigorously tested against existing software packages. 
pvalue and conf_int should be methods,
Do the inherited methods produce the correct result?

note: `use_t=True` as attribute chooses the t and F distributions in the default.

If df is an array (corresponding to df_resid), then the inherited inference methods might not work. Needs testing.