BUG: using pd.Grouper with a multi-level index and specifying level (and freq) buggy
http://stackoverflow.com/questions/25050015/resampling-over-dates-in-both-levels-of-a-multiindex-pandas-dataframe/25050074#25050074

```
from datetime import date
d0 = date.today() - timedelta(days=14)
dates = pd.date_range(d0, date.today())
date_index = pd.MultiIndex.from_product([dates, dates], names=['cohort_date', 'event_date'])
df = pd.DataFrame(np.random.randint(0, 100, 225), index=date_index)
```

Works

```
df.reset_index().groupby([pd.Grouper(key='cohort_date',freq='W'),pd.Grouper(key='event_date',freq='W')]).sum()
```

This should be equivalent, but raises

```
df.groupby([pd.Grouper(level='cohort_date',freq='W'),pd.Grouper(level='event_date',freq='W')]).sum()
```
