Prelim impl to change image on color-sku hover

(not shown: when user hovers in/out of sku options, fire a
refresh.sku.[color/size] event)

Minor refactor: rename some private functions, create new private
function changeSku, make init slightly more chainable (although I should
have done the proper thing and used this.each)

Add a destroy function to remove event listener (e.g. if used with
lightboxing, ensures that opening N lightboxes, we don't run this same
event handler N times -- memory/performance benefits?)