BUG: Make Styler ignore None index names to match to_html behaviour
Addresses https://github.com/pydata/pandas/pull/12090#issuecomment-180695902 by making the `Styler` behaviour match regular `to_html` behaviour. 

This PR is based from #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 
