title: Angular.js $watch on form/form inputs validity
body: <p>Is there any particular reason why this deep watch doesn't fire?</p>
  
  <p><a href="http://plnkr.co/edit/6FxMS4RlfljBMkprZYQs?p=preview" rel="nofollow">http://plnkr.co/edit/6FxMS4RlfljBMkprZYQs?p=preview</a></p>
  
  <p>Could it be that watch doesn't check angular attributes? Ie. those begging with $? If that's the case, is there any way to watch on validity change, and not just on the form, but on any of the inputs? </p>