Include aria-required attribute on required fields

This commit adds accessibility support for user-agents/screen readers that do not yet fully support HTML5 and its required attribute, [in particular IE9](http://www.w3.org/WAI/GL/wiki/Techniques/HTML5/Using_the_required_attribute_to_indicate_a_required_input_field#Screen_reader_support).

Reference on aria-required:
http://www.w3.org/TR/wai-aria/states_and_properties#aria-required
https://developer.mozilla.org/en-US/docs/Accessibility/ARIA/ARIA_Techniques/Using_the_aria-required_attribute

Closes issue #780