Supporting HTML 5 'formnovalidate' attribute

I was wondering if you were planning on supporting the html 5 'formnovalidate' attribute (http://www.whatwg.org/specs/web-apps/current-work/#attr-fs-formnovalidate). I think it would be a great addition for providing an html 5 compliant way for allowing submit buttons to bypass form validation.

This PR identifies a simple way of adding the support by duplicating your logic for excluding buttons with a 'cancel' class.

Thanks,
Paul