Rename props.current -> props.currentDomIndex, and add props.currentSlideIndex

Also add currentSlideIndex as a second param to the callback functions. It's my understanding that props.current referred to the index of the current slide within the DOM, hence the renaming.
Merge pull request #6 from kvcrawford/current_index

Rename props.current -> props.currentDomIndex, and add props.currentSlideIndex