Add role="button" to chosen-single class link

[juanolalla]

Summary

Adds role="button" attribute to <a class="chosen-single">. It also adds an aria-label to inform screen readers what will happen if you click the button, "Show options" or "Hide options".

Hides the visual arrow up/down indicator container from screen readers with aria-hidden.

Please double-check that:

• All changes were made in CoffeeScript files, not JavaScript files.
• You used Grunt to build the JavaScript files and tested them locally.
• You've updated both the jQuery and Prototype versions.
• You haven't manually updated the version number in package.json.
• If necessary, you've updated the documentation.

See the Pull Requests section of our Contributing Guidelines for more details.

References

Solves #52