BinGrouper AttributeError when returning a Series from DataFrame.apply
http://nbviewer.ipython.org/4118124/

``` python
import pandas as pd
from pandas.tseries.resample import TimeGrouper

N = 1000
ind = pd.date_range(start="2000-01-01", freq="D", periods=N)
df = pd.DataFrame({'open':1, 'close':2}, index=ind)
tg = TimeGrouper('M')
grouper = tg.get_grouper(df)
# Errors
df.groupby(grouper).apply(lambda df: df['close'] / df['open'])
# AttributeError: 'BinGrouper' object has no attribute 'groupings'
```

Error does not occur for regular Grouper. 
