Fix data-background-video autoplay on iOS Safari

print-pdf layout slide contents to fit stretch elements

print-pdf using callback iso timer

replace cdn.mathjax.org with cdnjs

cdn.mathjax.org is shutting down: https://www.mathjax.org/cdn-shutting-down/

This changes the default MathJax URL in the math plugin, as well as
references in README.md and test/examples/math.html

layout() and thereby layoutSlideContents() is never invoked
when isPrintingPdf() is true so stretch elements are not
sized correctly.
This is ensured now by invoking the layoutSlideContents
from SetupPDF(). There seems no need to scale down
like in other PR's is suggested.

Use window.callPhantom icw page. onCallback to wait for pdf-ready event.
From: http://stackoverflow.com/a/28925479/129269

Update to README.md for PDF printing

added toggleHelp function

this way a key can be given ability to toggleHelp on and off.  Previously Reveal.showHelp could open the help screen, but no way to close it.