add UMD support. fix #787
build reveal.min.js
fix #627
new api method: getBackgroundSlide
break showing/hiding of slides into separate methods
disregard v index when there is no vertical slides/backgrounds in getSlide/getSlideBackground
fix getindices test
lazy load all slide backgrounds
better defered loading of background media
getSlideBackground now works in pdf mode, add pdf tests
merge umd wrapping