Added custom swipe threshold option

In some situations it could be useful to be able to tune the amount of swipe distance required to go to the next/previous slide (expecially in a singleItem=true config).
To achieve this, I've added the option "slideDivider" and tweaked the function closestItem() in order to use it inside the $.each(...) block.

E.G. with the following configuration, the user would have to swipe left or right for at least 1/5 of items' width in order to go to the next/previous item:

$("#homeOwlCarousel").owlCarousel({
   ...
   slideDivider: 5,
   ...
});

PS: the name "slideDivider" is indeed a bit confusing, but I couldn't figure a better one for this!