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
Merge pull request #4 from zurb/master

Update Foundation
Merge pull request #5 from winghouchan/master

Update accessibility-classes Branch
Fix bad code
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
+ revert: some changes from https://github.com/zurb/foundation/commit/cc00c427f7cc76086c6d9aaff335ab70a9de45dd
Merge pull request #4992 from winghouchan/accessibility-classes

Add Accessibility Classes
added courses banner to grid.html
Merge branch 'master' of github.com:zurb/foundation into course-banner
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
Fix spelling errors

$visibility-hidden-list
Fix line height issue