Fix arrow down navigation on touch devices

When using the totalTime-based pacing calculation, a presenter may
inadvertently set totalTime and per-slide data-timing attributes in
such a way that the pacing time for some slides is impossibly low or
even negative.

Add a check to ensure that the pacing on a slide never falls below a
configurable minimum, defaulting to 0. Display an alert if the
pacing for any slide(s) falls below the threshold.

new API methods for getting horizontal/vertical slides, force linear navigation for one-dimensional decks

iframe background preload behavior now matches inline iframes + adheres to the new 'preloadIframes' config option

The current pacing timer operates on the assumption that there is
a default amount of time to be allocated to each slide, and that
individual slides can deviate from that default by specifying their
own data-timing attribute.

This patch introduces an alternate pacing method: by specifying
the totalTime configuration option, the presenter can set the total
time available to present. The pacing timer will then continue to
allocate the exact pacing time for slides that do have data-timing
set, as before. However, rather than applying the defaultTiming
constant to all others, it will

- Add up the time already allocated to slides via data-timing;
- subtract that from totalTime;
- divide the difference by the number of slides without data-timing set;
- apply the thus-calculated average to those slides.

totalTime has no default, and if both defaultTiming and totalTime are
set, totalTime wins. This preserves backward compatibility: if a
presenter has set defaultTiming and updates reveal.js, totalTime will
be null and defaultTiming is still applied to all slides without a
data-timing attribute. The presenter can then switch to the automatic
calculation, if desired, by setting a value for totalTime.

Bugfix: Remove calls to missing modules from gruntfile

When running the app, Grunt throws the following warnings:

>> Local Npm module "grunt-contrib-clean" not found. Is it installed?
>> Local Npm module "grunt-contrib-nodeunit" not found. Is it installed?

These modules don't appear to do anything except throw a warning so they can probably just be removed.

Allow Navigation Mode setting to change the shortcuts table

Allows lazy-loaded (i.e. data-src) iframes to be preloaded when they
come within the viewDistance, rather than once they're visible.

Fixed not bing able to follow same inner links twice with history:false