normed has been deprecated for matplotlib histograms but not for pandas
Historically, if you wanted a dataframe histogram to output a probability density function (as opposed to bin counts) you would do something like:

```python
df.hist(normed=True)
```

This falls in line with the old matplotlib style. However the new matplotlib style has deprecated `normed` and replaced it with `density`. 

I thought this should work automatically because `df.hist()` accepts kwargs, however it seems that this isn't working because it throws an attribute error.

```
AttributeError: Unknown property density
```

This behavior should probably be changed since `normed` is deprecated.