aper-ripple` provides a visual effect that other paper elements can
use to simulate a rippling effect emanating from the point of contact.  The
effect can be visualized as a concentric circle with motion.

Example:

    <paper-ripple></paper-ripple>

`paper-ripple` listens to "down" and "up" events so it would display ripple
effect when touches on it.  You can also defeat the default behavior and 
manually route the down and up actions to the ripple element.  Note that it is
important if you call downAction