mdCheckbox: focus should show ink circle

[marcysutton]

mdCheckbox has a temporary focus style where a dotted border is shown around the textContent. To follow the Material Design spec more closely, the checkbox should show an ink circle on focus that persists until the element is blurred:

This will make checkboxes labeled with only the aria-label and no textContent show focus. An example requiring this is the list item control by @rschmukler (#985, currently in progress):

<md-list>
    <md-list-item>
        <p>Subscribe</p>
        <md-checkbox aria-label="Subscribe"></md-checkbox>
    </md-list-item>
</md-list>

[marcysutton]

Question submitted around whether focus styles should persist when clicking with the mouse or trackpad, since the browser is actually focusing the control. Keyboard styles are definitely still needed (but they are already a WIP).