Validation for S3 UI fields

Closes #30 

Uses the new i18n system for error messages, and HTML5 form validation attributes.
I still don't get the point.  You can write a few lines of code to generate the list of errors based on the validation.

Also does that support &lt;paper-input>?  How nicely is it integrated?
That sample page has an error-box div to show the errors, while normal paper-input elements has 'error' attributes to be shown under the element.  They are really different, so wondering how beneficial is that library.