small fix for issue #814

fixed two minor bugs, a twohanded gesture may not have a direction, and if autoCenter is of, the tipPosition was in the wrong scope

renamed markdown attributes for clarity and consistency: data-vertical -> data-separator-vertical, data-notes -> data-separator-notes

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).

Correction of a small error