Add basic aria roles

Here's a PR that should cover the basics re aria-roles as per [issue #53](https://github.com/google/web-starter-kit/issues/53).
This is contained in a button, so you don't need the role here. The alt text is sufficient to provide a text alternative; the title attribute is redundant.

You can use Accessibility Developer Tools to help debug what the spoken text is likely to be: https://chrome.google.com/webstore/detail/accessibility-developer-t/fpkknkljclfencbdbgkenhalefipecmb?hl=en - use the Accessibility sidebar in the Elements panel while inspecting the <button> element.