Using .compound in groupby
``` python
df = pd.DataFrame({'grp': [0, 0, 1, 1], 'returns': [0, 0.1, -.1, .1]})
df.groupby('grp').returns.compound()
df.groupby('grp').returns.apply('compound')
```

Doesn't seem to work:

```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-193-cb3ce32c6c2e> in <module>()
      1 df = pd.DataFrame({'grp': [0, 0, 1, 1], 'returns': [0, 0.1, -.1, .1]})
----> 2 df.groupby('grp').returns.compound()
      3 df.groupby('grp').returns.apply('compound')

/home/wiecki/envs/zipline_p14/lib/python2.7/site-packages/pandas/core/groupby.py in __getattr__(self, attr)
    481             return self[attr]
    482         if hasattr(self.obj, attr):
--> 483             return self._make_wrapper(attr)
    484 
    485         raise AttributeError("%r object has no attribute %r" %

/home/wiecki/envs/zipline_p14/lib/python2.7/site-packages/pandas/core/groupby.py in _make_wrapper(self, name)
    496                    "using the 'apply' method".format(kind, name,
    497                                                      type(self).__name__))
--> 498             raise AttributeError(msg)
    499 
    500         # need to setup the selection

AttributeError: Cannot access callable attribute 'compound' of 'SeriesGroupBy' objects, try using the 'apply' method
```

apply also doesn't work.

This is with pandas 0.14.
