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. 
I think: 
save should be a string so we can save with different formats.
e.g. currently only `save='full'` is supported
use `save=None` for no saving