BUG: Make Styler ignore None index names to match to_html behaviour

Addresses
pandas-dev#12090 (comment) by
making the `Styler` behaviour match regular `to_html` behaviour.
This PR is based from pandas-dev#12162.    ##### New behaviour  <img width="596"
alt="screen shot 2016-02-08 at 10 35 23 am"
src="https://cloud.githubusercontent.com/assets/3064019/12890011
/b183e81e-ce4f-11e5-9b9f-c021fcb33c5a.png">    cc @TomAugspurger

Author: Tom Augspurger <[email protected]>
Author: Henry Hammond <[email protected]>

Closes pandas-dev#12260 from HHammond/style-remove-col-index-none and squashes the following commits:

48c2f4c [Henry Hammond] BUG: Ignore style column headers when None
a15248a [Tom Augspurger] ENH: display_format for style

Author: TomAugspurger