title: Aggregation fails when using lambdas
body: <p>I'm trying to port parts of my application from pandas to dask and I hit a roadblock when using a lamdba function in a groupby on a dask DataFrame.</p>
  
  <pre><code>import dask.dataframe as dd
  
  dask_df = dd.from_pandas(pandasDataFrame, npartitions=2)
  dask_df = dask_df.groupby(
                          ['one', 'two', 'three', 'four'],
                          sort=False
                      ).agg({'AGE' : lambda x: x * x })
  </code></pre>
  
  <p>This code fails with the following error:  </p>
  
  <p><code>ValueError: unknown aggregate lambda</code>  </p>
  
  <p>My lambda function is more complex in my application than here, but the content of the lambda doesn't matter, the error is always the same. There is a very similar example in the <a href="http://dask.pydata.org/en/latest/dataframe-api.html?highlight=agg#seriesgroupby" rel="noreferrer">documentation</a>, so this should actually work, I'm not sure what I'm missing.   </p>
  
  <p>The same groupby works in pandas, but I need to improve it's performance.</p>
  
  <p>I'm using dask 0.12.0 with python 3.5.</p>