Auto-sync children key models if $sync is called on the parent

[colinmacdonald]

$scope.todos = $goKey('todos').$sync();

$scope.todos.$on('ready', function() {
  var myTodo = $scope.todos.$key('id-1231231adasd-000');
  // myTodo.desc === 'test test'
  // myTodo.complete === false
});

We already have this data sync'd on the parent, should make it easier to work with child key models.

[colinmacdonald]

This could be quite useful if using with $save on a specific child node (when we have it) instead of having to $sync first. Thinking something like:

  $scope.todos = $goKey('todos').$sync();

  $scope.editTodo = function(id, desc) {
    var todo = $scope.todos.$key(id);
    todo.desc = desc;
    todo.complete = !todo.complete;

    todo.$save();
  };

to prevent children from being sync'd automatically we could have an option on either the parent sync

  var todos = $goKey('todos').$sync({capture: true}); // default capture === false

or when creating a new model on a child

  var todo = todos.$key('id-1231y182u12b-000', { sync: true/false });