Automatically generate visibility classes

Replaces most of the visibility classes with a mixin that simplifies
the process of adding or removing breakpoints and their associated
visibility classes. Resulting CSS is comprehensive without containing
any extraneous or nonsensical classes, and is far less error-prone.
Made style adjustments to new visibility functions
Separates legacy visibility classes in dynamic visibility class generator loop

Removes functionality to omit visibility classes based on relative
size, in favor of generating consistent classes. Separates legacy
visibility so they can be omitted.
Isolates legacy visibility classes to legacy breakpoints only

The .show/hide-for-[size] and .show/hide-for-[size]-down classes are
now only generated for small, medium, and large breakpoints, and only
if legacy visibility classes are enabled.
Orbit fix / Run images_loaded callback if images.length == 0

This caused the orbit slider to not load/init. If orbit is used as content slider and not as images slider than the callback image_loaded in L287 (foundation.orbit.js) is never fired.

So the callback needs also to be fired if the image array is empty.

Related issues:
http://foundation.zurb.com/forum/posts/2144-orbit-not-visible-on-load-when-interchange-used-for-images
replace .children('img') with .find('img')

Don't expect images to be direct children of the slide element item. An image could also have an ```<a>``` tag as wrapper.
Removed repeated IDs in Kitchen Sink demo

Tabs in Accordion demo’s first panel was sharing IDs with the tabs demo
Fixing #4397

Configuration of sub-nav radius is not scoped since it is using $global-radius. Fix is to consume $sub-nav-border-radius instead of $global-radius.
Merge pull request #4383 from christianseel/patch-3

Orbit fix / Run images_loaded callback if images.length == 0
Merge pull request #4388 from th3byrdm4n/master

Removed repeated IDs in Kitchen Sink demo
Merge pull request #4398 from ohenak/patch-1

configuration of sub-nav radius is not scoped.
Resolved conflicts
Added visibility class html variables to settings file.
Merge pull request #4422 from zurb/alandaguegreene-alandaguegreene-dynamic-visibility-classes

Added alandaguegreene's dynamic visibility classes
Fixed a formatting issue on the kitchen sink page
Fixed some kitchen sink assets to correctly demo the block grid