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. 
`go` doesn't sound like a good name

maybe:
use `method=None` as indicator that we don't want to create imputers automatically, and add manually (through methods.