BUG: dont ignore use_index kw in bar plot

Closes https://github.com/pydata/pandas/issues/6209

Here's the fixed output for the same example from the issue: `df.plot(kind='bar', use_index=False)`:

![line_false_fixed](https://f.cloud.github.com/assets/1312546/2053850/603f044e-8aad-11e3-86e3-3f3bf38a5683.png)

I thew it in `.13.1`, can push to `.14` if need be.