Control bar text

There is a bug, causing `chart.stacked(true)` to be ineffective, and user has to click two times in order to update the chart:
![image](https://cloud.githubusercontent.com/assets/1113178/3404058/dcec4e66-fd6c-11e3-9617-9d12689eff60.png)

Click Stacked, then Grouped, then Stacked, and graph updates.
The issue is most likely here: https://github.com/ividata/nvd3/blob/a0544ea651371590dd9aa2dc2490cdc5150779bf/src/models/multiBarHorizontalChart.js#L36, the `disabled` is not updated when we programmatically set `stacked` - only when user clicks.