ENH: support Categorical hist plotting
```
s = Series(list('abbc')).astype('category')
```

This raises

```
s.hist()
```

This works

```
s.value_counts().plot()
```

I think this should work, beyond this, anyone have ideas on what/how to plot categorical data?

cc @JanSchulz
cc @TomAugspurger 
cc @jorisvandenbossche 
