Fix infinite ticks when using negative multiplier

[stephg]

I was using impetus to update scrollLeft and scrollTop of a parent container and thus using a negative multiplier to invert scroll direction.

Then the stop condition for requesting new animation frames was never hit. destroy() can't stop it either - that would be worth a fix too (#31).

[GianlucaGuarini]

This patch solves a really important issue. @chrisbateman have you any plan to merge it anytime soon? Thanks