Re-implementing moveNext and movePrevious with ko

Just the sliding left/right for now through a binding on the container's
css class which gets set on moveNext/movePrevious and reset when the
transition ends.

TODO...
- switching around the items (dropping one off one end, adding one to the other)
- fading in/out those items
- setting the current item
- make sure I'm handling the transition ends properly (i.e., ignore child transition ends)
...maybe I can do this with ko.dataFor(event.target) or ko.contextFor(event.target) in
the completeTransition function?