title: Pandas aggregate for counting with nans
body: <p>The pandas count aggregate ignores nan's. I need a count that includes them. Numpy has aggregates for some but not all nan modified aggregates, do I have to use a custom aggregate or is there a way doing this that I can't find?</p>
  
  <p>This is for groupby's, and I want the normal NaN functionality for mean, but weird for count. In code</p>
  
  <pre><code>In [1]: import numpy
  
  In [2]: import pandas as pd
  
  In [3]: df = pd.DataFrame([[0,float('nan')],[0,float('nan')],[0,float('nan')]])
  
  In [4]: df.groupby(0).agg(['count', 'mean'])
  Out[4]:
        1
    count mean
  0
  0     0  NaN
  </code></pre>
  
  <p>I want the output to be 3 Nan instead of 0 NaN.</p>