Add border on currently selected tool.

Using polymer's core-selection element.
Place a turquoise blue border around currently selected tool.
Adjust margin of selected class to ensure that size of selected element
including border remains the same size as the non-selected elements.

BUG=https://github.com/jackhou-chromium/bitmapper/issues/30
R=tapted@chromium.org

Review URL: https://codereview.appspot.com/191270043