add ie10 support to form validation

fixes #1057

[This is how microsoft demos it](http://ie.microsoft.com/testdrive/HTML5/Forms/) (go to `New DOM Properties and Events`)

They are just adding an onclick handler to the submit button, that runs `checkValidity()` on each input element.

```javascript
for (var i = 0; i < form.elements.length; i++) { 
  if (!form.elements[i].checkValidity()) {
    formValidity = false;
  }
}
```