Horizontal barplot
I just want to flip the direction of the bars in a barplot, but currently, the orientation of a barplot cannot be set to 'horizontal':

```
pd.Series([1,2], index=pd.Index([1,2], name='x')).plot(kind='bar', orientation='horizontal')
```

yelds an:

```
AssertionError: incompatible sizes: argument 'left' must be length 1 or scalar
```
