mdButton: Keyboard pressed state should work similarly to mouse click

[marcysutton]

Although there is a focus state, buttons have no visible pressed state when using the keyboard. They should have some visual feedback similar to a mouse click.

[ThomasBurleson]

At a minimum, ripple effects should be trigger for spacebar/enter keypresses

[Aaronaught]

Enter/space triggering the ripple from the center of the button is exactly how it works in the Polymer/Paper counterpart.

Is there a workaround in the meantime - some way to programmatically trigger the ripple? Otherwise it's somewhat of a usability/accessibility problem.