make validatr conform to the html5 spec when form elements are disabled

Currently validatr does not conform to the HTML5 for validation of form fields when some of those fields are disabled. See this section of the HTML5 specification:

http://www.w3.org/html/wg/drafts/html/master/forms.html#enabling-and-disabling-form-controls:-the-disabled-attribute

This pull request adds code to ensure that disabled elements are not checked for validity, allowing the form to be submitted.