added code pen integration

Added code pen integration, represented by a pencil icon on the demo snippet.

Should work out of the box for any component without sub-components.

any subcomponents need to add a parent-element attribute to the px-demo-snippet tag, calling their parent component.

Please note that at this point, any component with polymer-font-awesome will NOT show up the icons, since polygit expects a px- file, and polymer-font-awesome.html does not begin with one.