Gum overlay logic

I did some rebasing and used git push force and I managed to loose the reference to the original pull request #229.

Anyway I've fixed the changes based on the comments in #229 and made a clean pull request.

Sorry for the hassle.

Fixes #137 .
I mean by modifying the html and have the button start disabled.
     <paper-button id="start-button" onclick="start()" raised disabled>