DOC: Fix table styling in main docs
When switching to nbsphinx, I modified the site's CSS so
that the converted notebook looks decent. This had some
unfortunate changes on tables elsewhere in the notebook.

This change fixes the headers to be left-aligned in the main site,
and right-aligned for the tables generated by `df.style` in the
nbsphinx-converted notebook.

xref https://github.com/pandas-dev/pandas/pull/15581/

Before:
<img width="801" alt="screen shot 2017-04-26 at 7 11 19 am" src="https://cloud.githubusercontent.com/assets/1312546/25437376/c568c4b2-2a5b-11e7-9bb0-f524d87d3813.png">


After:

![screen shot 2017-04-26 at 8 23 43 am](https://cloud.githubusercontent.com/assets/1312546/25437380/cba5a07a-2a5b-11e7-9669-e9e469e3a7e7.png)
