Include `from` and `to` indexes in Carousel `slide` event

Created by: jimmydo

The slide event callback will now receive the indexes of the items that the carousel is transitioning from and to as the second and third arguments, respectively.

Example:

someCarousel.bind('slide', function (event, fromIndex, toIndex) {
...
});

I chose to pass the indexes instead of the actual DOM elements, to be consistent with the .carousel(number) function, which uses a zero-based index to the desired carousel item.

This should also address issue #2240 (closed).