BUG: GroupBy.get_group doesnt work with TimeGrouper

`get_group` raises `AttributeError` when the group is created by `TimeGrouper`.

```
>>> df = pd.DataFrame({'Branch' : 'A A A A A A A B'.split(),
                   'Buyer': 'Carl Mark Carl Carl Joe Joe Joe Carl'.split(),
                   'Quantity': [1,3,5,1,8,1,9,3],
                   'Date' : [
                    datetime(2013,1,1,13,0), datetime(2013,1,1,13,5),
                    datetime(2013,10,1,20,0), datetime(2013,10,2,10,0),
                    datetime(2013,10,1,20,0), datetime(2013,10,2,10,0),
                    datetime(2013,12,2,12,0), datetime(2013,12,2,14,0),]})

>>> grouped = df.groupby(pd.Grouper(freq='1M',key='Date'))
>>> grouped.get_group(pd.Timestamp('2013-12-31'))
AttributeError: 'DataFrameGroupBy' object has no attribute 'indices'
```