BUG/API: autocorrelation_plot should accept kwargs
Unless there's a specific reason not to for this plot?  All the others seem to accept them.

``` python
autocorrelation_plot(data, label='ACF')
```

![label_autocorr](https://f.cloud.github.com/assets/1312546/1649385/4cad26d2-59fb-11e3-9d3e-2ccc7c44856e.png)
