Change the file structure of the project to follow Polymer's

- Follow the structure proposed in the `seed-element` in [Creating reusable elements on Polymer's site](http://www.polymer-project.org/docs/start/reusableelements.html).
- Change default value in the demo to `50` so it is more clear what it does.
- Depend on latest version of Polymer.
- Increase major version of the component.