Closes #1226, #1538.

Add an option to display advice on whether the current pace of the
presentation is on track for the right timing (shown as green), and if
not, whether the presenter should speed up (shown as red) or has the
luxury of slowing down (blue).

The pacing timer can be enabled by configuring by the `defaultTiming`
parameter in the `Reveal` configuration block, which specifies the
number of seconds per slide.  120 can be a reasonable rule of thumb.
Timings can also be given per slide `<section>` by setting the
`data-timing` attribute.  Both values are in numbers of seconds.

When the option is enabled, clicking on the timers will reset the timer
to the beginning of the current slide, i.e. as if pacing was perfectly
on track, not to zero as if the presentation had just begun.

and make muting work for negative values

This allows us to use different colours for different timers,
which will be useful when we add a pacing timer.

This will allow us to reuse the display code for displaying
an additional pacing timer.

This can be used by plugins which are concerned with helping the
presenter with their pace.

This can be used by plugins which are concerned with helping the
presenter with their pace.

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