title: groupby for pandas Series not working
body: <p>I am unable to do a groupby on a pandas Series object.  DataFrames are fine, but I cannot seem to do groupby with a Series.  Has anyone been able to get this to work?</p>
  
  <pre><code>&gt;&gt;&gt; import pandas as pd
  &gt;&gt;&gt; a = pd.Series([1,2,3,4], index=[4,3,2,1])
  &gt;&gt;&gt; a
  4    1
  3    2
  2    3
  1    4
  dtype: int64
  &gt;&gt;&gt; a.groupby()
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/generic.py", line 153, in groupby
      sort=sort, group_keys=group_keys)
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 537, in groupby
      return klass(obj, by, **kwds)
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 195, in __init__
      level=level, sort=sort)
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 1326, in _get_grouper
      ping = Grouping(group_axis, gpr, name=name, level=level, sort=sort)
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/groupby.py", line 1203, in __init__
      self.grouper = self.index.map(self.grouper)
    File "/share/apps/install/anaconda/lib/python2.7/site-packages/pandas/core/index.py", line 878, in map
      return self._arrmap(self.values, mapper)
    File "generated.pyx", line 2200, in pandas.algos.arrmap_int64 (pandas/algos.c:61221)
  TypeError: 'NoneType' object is not callable
  </code></pre>