Fixes bad NaN applied to style on updateParallax.

The previous criteria "verticalSlideCount > 0" would result
in verticalOffset being NaN when verticalSlideCount == 1.

This would cause dom.background.style.backgroundPosition to
be set to something like "123px NaNpx", ultimately preventing
the parallax effect to play (silently failing so far).
server side notes plugin now supports input via data-notes attribute
renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes
Merge branch 'master' of https://github.com/ccortezia/reveal.js into dev