see https://github.com/hakimel/reveal.js/issues/929#issuecomment-80738837

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

grammar

background video loop

It's possible for slides to be in a situation where the last clicked thing was an SVG before the tab/window loses focus. When returning, `.blur()` is called on the previously-active element, but can result in an exception.

This protects against that and will only call `.blur()` when `document.activeElement` supports it.

Refactored overview mode

themes grunt task was renamed