Cherry picking from branch fix-fragments-in-speakernotes

Change source from reveal.min.js to reveal.js for testing purposes.

Add nextFragment and previousFragment to the API:
Making it possible to call the methods Reveal.nextFragment()
and Reveal.previousFragment() from »outside«.

Update README.md:
Add the new API Methods Reveal.prevFragment() and Reveal.nextFragment()

js/reveal.js

@@ -1316,6 +1316,8 @@ var Reveal = (function(){
 		down: navigateDown,
 		prev: navigatePrev,
 		next: navigateNext,
+		prevFragment: previousFragment,
+		nextFragment: nextFragment,
 
 		// Deprecated aliases
 		navigateTo: slide,