Adding accessibility support

Adds aria role of ```progressbar``` and exposes an ```alt``` attribute that will be used as ```aria-label```.
```alt``` defaults to ```loading``` if not specified, not great for internationalization but in my opinion better than the alternative which would make it hidden from screen readers.
Also setting ```aria-hidden``` when active state changes.

Just to clarify the logic - if `this.alt === ''`, then the spinner is purely decorative, so the screenreader shouldn't see it?