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.

Yes, that's correct. I can see that use-case coming up if author has some related visible text after the loading indicator. Like ```<paper-spinner/> Loading your feed....``` in which case it's fine to mark the spinner purely as decorative. Similar to <img> however I think the decision to hide the spinner should be very explicit, hence the requirement to explicitly set it to empty string rather than not defining it.