Return percentage of Null values in pd.Series.isnull()
#### Problem description
One thing I have to check a lot at my work is the percentage of null values in a DataFrame column. 
What I end up doing is 
 ```python 
(df[column].isnull().sum() * 100/ len(df)).sort_values(ascending=False)
```

I think it would be very convenient to have a parameter, like:

```python
df[column].isnull(ratio=True,sort=True)
```

What do you guys think?
