`<tbody/>` is automatically inserted, so we can guarantee that this affects all ths and tds in the last row of the table *body* and the head is left untouched

The alignment information were already in the generated html.
The css was the only thing missing, so I added it.

css/theme/template/settings.scss adds the $headingMargin variable, set to `0 0 20px 0`.
css/theme/template/theme.scss uses $headingMargin to set margin on headers.

reveal.js was lacking a nice, simple, minimal theme. now it has one.
bam.

the old one was pink. while I don't have anything against pink, I just thought it might be good to make the highlight color dark brown instead.

themes are now available as separate css files under css/theme/, old theme option in config is no longer supported (closes #104)