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.
The disabled checkbox additionally needs a grey border and grey background if checked: http://d.pr/i/hj0S

Also, the `no-drop` cursor isn't being applied to the `.material-container` child, since that has its own `cursor: pointer` rule.

I don't know why the `cursor` rule was added to the `.material-container` child class. It should inherit its pointer from the main `material-checkbox` element., and not have its own rule. Could you change that?