grouped.filter with dropna option
in Pandas 0.16.2:

```
grouped = a.groupby(idx)
grouped.filter(lambda x: x.mean()>0.0, dropna=False)
```

in case there was no group whose `x.mean()` was larger than zero, an `AttributeError: 'list' object has no attribute 'astype'` would be raised (rather than returning an all `NaN` dataframe/serie in previous versions).
