Allow handles to be inherited properly.

So now you can set handles on e.g. the router and a single controller and it'll wait on _both_ of them.

E.g.:

```js
Router.configure({ 
  waitOn: function() { return Meteor.subscribe('something'); }
});

Router.map(function() {
  this.route('posts', {
   waitOn: function() { return Meteor.subscribe('somethingElse'); }
  });
```