code: func (fb *Firebase) ChildRemoved(fn ChildEventFunc) error {
  	return fb.addEventFunc(fn, fn.childRemoved)
  }
docs: // ChildRemoved listens on the firebase instance and executes the callback
  // for every child that is deleted.
  //
  // 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.