Remove hard coded border property in pandas `to_html()`

Hi,

https://github.com/pydata/pandas/blob/e4cb0f8a6cbb5f0c89b24783baa44326e4b2cccb/pandas/core/format.py#L893

Currently, a `border="0"` seems to be hard coded in the pandas `to_html()` table output. I think it'd be more convenient to have that as a CSS property on `dataframe` than put directly like that.
