title: How to watch object on unknown scope parent?
body: <p>I'm writing an element directive which can be used inside other directives.
  The structure of the parents containing the directive element is unknown.</p>
  
  <p>My directive needs to watch an object called 'modelState' which resides somewhere along the parents scopes tree.
  However, I can't tell which parent, which is why <code>scope.$watch("$parent.modelState", ...);</code> will now work at all times.</p>
  
  <p>How can I create this watch without knowing who the parent I need is?</p>