feat(ngAria): New module to make a11y easier

Adds various aria attributes to the built in directives.
This module currently hooks into ng-show/hide, input, textarea
button as a basic level of support for a11y. I am using this as a
base for adding more tags into the mix for form direction flow,
making ng-repeat updates atomic but the tags here are the most
basic ones.

Closes #5486 and #1600
`aria-checked` could also apply to a radio button and possibly other widgets http://www.w3.org/TR/wai-aria/states_and_properties#aria-checked