Closes #662.

use 'url()'-free path when specifying parallax image, refactor so that parallax is applied to background class, remove unused attributes #595

modified:   css/reveal.min.css
modified:   plugin/leap/leap.js

modified:   css/reveal.css
modified:   css/reveal.min.css
modified:   plugin/leap/leap.js

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.

They looked a little wonky if you used a G or a Y.

The Serif theme was a little flaky previously