TimeGrouper on a DataFrame
Am I using this incorrectly? (see [so qustion](http://stackoverflow.com/questions/16982370/pandas-combine-timegrouper-with-another-groupby-argument)).

```
In [1]: df = pd.DataFrame([[1, 2]], pd.date_range('20000101', periods=2))

In [2]: df
Out[2]:
            0  1
2000-01-01  1  2
2000-01-02  1  2

In [3]: g = df.groupby(pd.TimeGrouper('20D'))
```

Doesn't have some attributes, like group and friends:

```
In [4]: g.groups
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-b207e9b2ead3> in <module>()
----> 1 g.groups

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/groupby.pyc in __getattr__(self, attr)
    234
    235         raise AttributeError("'%s' object has no attribute '%s'" %
--> 236                              (type(self).__name__, attr))
    237
    238     def __getitem__(self, key):

AttributeError: 'DataFrameGroupBy' object has no attribute 'groups'

In [5]: g.get_group(0)
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-7ed4a1924eee> in <module>()
----> 1 g.get_group(0)

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/groupby.pyc in get_group(self, name, obj)
    270             obj = self.obj
    271
--> 272         inds = self.indices[name]
    273         return obj.take(inds, axis=self.axis)
    274

/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages/pandas/core/groupby.pyc in __getattr__(self, attr)
    234
    235         raise AttributeError("'%s' object has no attribute '%s'" %
--> 236                              (type(self).__name__, attr))
    237
    238     def __getitem__(self, key):

AttributeError: 'DataFrameGroupBy' object has no attribute 'indices'
```

Related to #1057 ?
