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
That is an useful attribute where `ng-disabled` is used to disabled certain fields based on the state of the form like so:
```
<form name="myForm">
    ...
   <input type="submit" ng-disabled="!myForm.$valid">
</form>
```
More info on `aria-disabled` : http://www.w3.org/TR/wai-aria/states_and_properties#aria-disabled