Paper tab listens for down event and explicitly fires ripple down/up.

This pull request allows us to interact with developer's tab content as a developer would expect removing the `pointer-event: none` currently applied and explicitly calling the ripple's down/upAction with the current event. 

Specifically, this allows us (me) to use html anchor tags within the paper-tabs' content to take advantage of the browser's natural navigation system (and avoid an unnecessary click listener just to update window.location). I'm using it, and thought I would share.