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

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.

For a fragment: allow to show a separate note defined in it

 	Turn off preview links locally

support .sass themes