BUG: _repr_html_ doesn't use `display.colheader_justify`
![screen shot 2014-05-25 at 9 59 50 am](https://cloud.githubusercontent.com/assets/1312546/3077252/416a4118-e41d-11e3-92e8-885eb0d31e99.png)

It is used correctly in `str(df)` and the terminal repr; I think just `_html` is the problem. I'll see if I can track it down quick.
