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. 
add something like an `add_imputations` method if we change our mind and want to increase the number of imputations (Stata has the option to add or redo the imputations.)