Add Accessiblity Classes

Add Accessibility Classes. `.hidden` hides element but allows screenreaders to read. Do not confuse with `.hide`. `.visible` reverses effect of `.hidden`. Less confusion with `.show`.

See https://github.com/zurb/foundation/issues/3597 for some background information.

Many thanks to @mmikitka for starting to improve accessibility and for defining the mixin used here. The rules within that mixin come from here: http://snook.ca/archives/html_and_css/hiding-content-for-accessibility
Fix bad code
Update _visibility.scss
Fix bad code
Revert to original master
Add New Accessibility Classes

Update Visibility Docs For New Accessibility Classes.
Create examples_accessibility_hidden.html

Add new example for new accessibility classes.

Still needs to be finished. :D
Create examples_accessibility_visible.html

Add new docs for new accessibility classes.

Still needs finishing. :D
Finish examples_accessibility_visible.html
Update visibility.html
Merge pull request #5030 from winghouchan/accessibility-visibility-docs

Update Docs for New Accessibility Classes