GroupBy[Index] raises TypeError
```
In [5]: means_by_hour_of_day = training_data.groupby('hour')[target_names].mean()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/Dropbox/book/svn/data/air_quality/<ipython-input-5-141d964fe4bb> in <module>()
----> 1 means_by_hour_of_day = training_data.groupby('hour')[target_names].mean()

/home/wesm/code/pandas/pandas/core/groupby.pyc in __getitem__(self, key)
   1663                                     as_index=self.as_index)
   1664         else:
-> 1665             if key not in self.obj:  # pragma: no cover
   1666                 raise KeyError(str(key))
   1667             # kind of a kludge

/home/wesm/code/pandas/pandas/core/frame.pyc in __contains__(self, key)
    566     def __contains__(self, key):
    567         """True if DataFrame has this column"""
--> 568         return key in self.columns
    569 
    570     #----------------------------------------------------------------------

/home/wesm/code/pandas/pandas/core/index.pyc in __contains__(self, key)
    263 
    264     def __contains__(self, key):
--> 265         hash(key)
    266         # work around some kind of odd cython bug
    267         try:

/home/wesm/code/pandas/pandas/core/index.pyc in __hash__(self)
    271 
    272     def __hash__(self):
--> 273         return hash(self.view(np.ndarray))
    274 
    275     def __setitem__(self, key, value):

TypeError: unhashable type: 'numpy.ndarray'
```
