title: pandas groupby ignoring NAs
body: <p>I have a DataFrame which has a lot of NAs.  pandas's groupby operation is ignoring any combinations with NA in it.  Is there a way to include NAs in groups?  If not, what are the alternatives to pandas groupby?  I really don't want to fill in NAs because the fact that something is missing is useful information.</p>
  
  <p>Edit: I noticed that my question is exactly the same issue reported in <a href="https://stackoverflow.com/questions/18429491/groupby-columns-with-nan-missing-values">groupby columns with NaN (missing) values</a>
  Has there been any developments technology to get around this issue?</p>