ENH: add parameter to to_html for HTML border
Closes https://github.com/pydata/pandas/issues/11563

Previously this was hardcoded to `<table border="1" ...>`. Now we allow an option to `to_html`.

The only controversial bit here may be the option. I really don't like having options control output, but given that this is essentially a repr I think it's OK. It's also the only way to expose it to the user since `_repr_html_` can't take any arguments. I also selfishly need this [for my own library](https://github.com/tomaugspurger/stitch), where the user wouldn't be able to pass an option 😄 

Interestingly, the `border="0"` is seemingly redundant in the notebook:

![screen shot 2016-08-21 at 7 51 02 pm](https://cloud.githubusercontent.com/assets/1312546/17840966/c4942c98-67d8-11e6-8359-90f46dbb8cb7.png)

Looking at the CSS, I see jupyter applying a border-width of 0.909091px in their style, though I can't remember if I've adjusted the default CSS on this computer; will check later.
