Add missing border styles in default stylesheet

This is for asciidoctor/asciidoctor#569.

I've never used sass before and I'm by no means a CSS expert so bare with me. It took a little trial and error to get things looking OK. I'm pretty happy with how it turned out and tables look a heck of a lot better in print view too. Other than copying the styles verbatim from epub3.css, there are a few small changes worth pointing out here:

1) I added a CSS selector block not found in epub3.css to avoid the `grid-all` class from adding a border on the right side of the table. My guess was that the `grid-all` class was intended to only style inner borders and the `frame-all` class to only style outer ones? The issue this addresses was noticeable when `frame-none` and `grid-all` were used in combination. If this is desired we may want to apply this fix to epub3.css too.

`table.grid-all td:last-child {
 border-right-width: 0;
}`

2) I tried to be clever and use the sass variables `$table-border-highlight-size` and `$table-border-highlight-color`. One consequence of this is that the color is `#dddddd` rather than `#80807F` if I had copied verbatim from the epub3.css.

3) Lastly I removed the original `table.grid-all` selector block that among other things was rounding the table corners. I noticed it wasn't used in epub3.css and it was causing me some grief so I got rid of it.

Let me know what you think. If any of this requires fixing up let me know. 

I suspect once this is merged we'll want a PR that copies the generated `asciidoctor.css` into the asciidoctor project?