Add aria-label and alt support

Original PR was eaten by git :\

This adds a `role` to core-icon and allows the developer to apply either `aria-label` or `alt` attributes to support screen readers. As a last ditch fallback it will try to include the icon name if one was provided.

cc @alice
Why not just add `alt` to the list of attributes on the `<polymer-element>` and make it first-class like `src` and `icon`?