Update to Polymer 0.5.0, add documentation and demo

Not sure if this element is still being maintained, but I've been using it in [one of my projects](https://github.com/Scarygami/cookie-consent) so I thought it would be nice to have it updated to the current Polymer version. Also added documentation and a demo page to go along with the current default layout for custom elements.

If you are not planning to maintain this package any longer, I'll just keep working in my fork and use it as dependency instead.