Fix arrow down navigation on touch devices

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

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

Scenario:

- history set to false in Reveal’s config
- follow anchor to `/#someid`
- navigate to other sections
- follow anchor to same section `/#someid`

This does not work as the hash has not changed.

This fix removes the hash from the URL when navigating