'mode' not recognized by df.groupby().agg(), but pd.Series.mode works
This works:

```
df = pd.DataFrame({'A': [1, 2, 1, 2, 1, 2, 3], 'B': [1, 1, 1, 2, 2, 2, 2]})
df.groupby('B').agg(pd.Series.mode)
```

but this doesn't:

```
df.groupby('B').agg('mode')

...
AttributeError: Cannot access callable attribute 'mode' of 'DataFrameGroupBy' objects, try using the 'apply' method
```

I thought all the series aggregate methods propagated automatically to groupby, but I've probably misunderstood?
