Add ripple effect to palette cells.
Add ripple effect to palette cells.

This is done by changing them from divs to paper-buttons.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/41
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/187670043

jackhou-chromium/bitmapper,jackhou-chromium/bitmapper,jackhou-chromium/bitmapper