Fixed issue where animation timer could be deallocated but not nilled

Using iCarousel in an ARC based project (carousel.type =
iCarouselTypeCoverFlow2), I was encountering a situation where the
animation would stop after my app loaded some data and did some
processing. After looking at the issue, I found the animation timer was
being deallocated but not nilled, breaking the logic in startAnimation.

Fix should work for ARC and non-ARC projects.