Update to Polymer 2.0

This amazing upgrade done by the talented @mkazlauskas at @firmfirm!

• Upgraded to use Polymer 2.0

Encouraging better use

Breaking changes in this release:

The input and valid properties have been removed. Instead, validation should be checked using validate() on the iron-input element. This allows one validator to handle multiple elements if need be.

Docs and demo have been updated to use this workflow.

Initial release

First one out the door.