Segfault in .groupby() with empty groups
I get a segfault when running:

pandas.DataFrame([1,2,3]).groupby([]).groups

As an aside, in my scenario (which I don't know how general it is) the user can supply the grouping. If no grouping should take place (i.e. groupby([])) I would like .groupby() to return the whole data, instead there is a:

/usr/local/lib/python2.7/dist-packages/pandas/core/groupby.pyc in _get_compressed_labels(self)
    547                 group_index = get_group_index(all_labels, self.shape)
    548             else:
--> 549                 group_index = all_labels[0]
    550             comp_ids, obs_group_ids = _compress_group_index(group_index)
    551             return comp_ids, obs_group_ids

IndexError: list index out of range

I'm not sure whether it would be sensible to change the behavior so that the whole DataFrame is returned, but to me it would be more useful than raising an error.
