Made custom Polymer element to encapsulate the color palette. Color palette now refers to its selected div based on an
action being fired.

Styling also changed to fit in with new UI; palette cells are
circular and match the size of the tool buttons.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/20
BUG=https://github.com/jackhou-chromium/bitmapper/issues/22
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/185700043