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. 
If we get more keywords for the details of a method, then I think `method_kwds` would be better as argument, instead of k_pmm or whatever method arguments we need.
It would be more flexible to extend without changing the numbers of arguments to the method. Although a bit more tedious to write `method_kwds={'k_pmm':1}`

example for possible extensions: if we get different matching algorithms or if we get different bootstrap methods.