enh: api cleanup... tostring, to_string, to_html, etc.
There is a lot of semi-duplication / differences in the `Series()` and `DataFrame()` methods.

Maybe it's worth taking a look through them and regularising.

Fwiw, I prefer  `tostring` vs. `to_string`

Also, would be nice to have `tohtml` on things, though expanding it is beyond the scope of this.
