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.
No... since this is just a 0.0.2 release (on Tuesday). 

The outstanding ink bugs and parity issue(s) can simply be listed as known and `in-progress` issues. 
This keeps our planning and tasks more manageable.

> Also I need to consider how to reuse the ripple canvas for focus features.