Accessible checkbox

Users should be able to to interact with `material-checkbox` components using a screen reader and keyboard. These changes make those components into interactive elements by adding `tabIndex` and visual feedback on focus, the ARIA `checkbox` role and a dynamically-updated `aria-checked="[ true | false ]"` attribute.
For focus styles, we need the ripple that stays: http://d.pr/i/hj0S

I think we should hold off on focus styles until the focus ripple is ready. The Google UX team probably wouldn't be happy with the dotted border, even as an interim style. @ThomasBurleson do you think the focus ripple could be done by Tuesday?