BUG: in .groupby.SeriesGroupBy plot not accessible?
Previously, in `0.12` and earlier, I could quickly visualize `groupby` objects with a call to `.plot()`. Currently in master, the `.plot` method on a `groupby` object raises, with following error:

``` python
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-63-3760a47a721b> in <module>()
----> 1 grp.kWh.plot()

/pandas/pandas/core/groupby.pyc in __getattr__(self, attr)
    250 
    251         if hasattr(self.obj, attr) and attr != '_cache':
--> 252             return self._make_wrapper(attr)
    253 
    254         raise AttributeError("%r object has no attribute %r" %

/pandas/pandas/core/groupby.pyc in _make_wrapper(self, name)
    265                    "using the 'apply' method".format(kind, name,
    266                                                      type(self).__name__))
--> 267             raise AttributeError(msg)
    268 
    269         f = getattr(self.obj, name)

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