code: func (fb *Firebase) ChildAdded(fn ChildEventFunc) error {
  	return fb.addEventFunc(fn, fn.childAdded)
  }
docs: // ChildAdded listens on the firebase instance and executes the callback
  // for every child that is added.
  //
  // You cannot set the same function twice on a Firebase reference, if you do
  // the first function will be overridden and you will not be able to close the
  // connection.