Upgraded Interactive Demo from Web-UI to Polymer

This PR upgrades the demo from webui to polymer.

I also have the code to make this deploy automatically on every commit. [You can see a running version here.](http://caffinatedmonkey.github.io/dart-markdown/)

Since this is only a demo and not a full application, I removed the css and html highlighting.

Review: @dpeek, @kevmoo

Closes #24