code: function Callback (connection, callbackId, callbackUrl, nuki) {
    EventEmitter.call(this);
  
    this.connection = connection;
    this.nuki = nuki;
    this.callbackId = callbackId;
    this.url = callbackUrl;
  }
docs: The constructor for callbacks via bridge or nuki.
  
  If called from nuki it will emit on it and check the battery level,
  otherwise (called from bridge) it will only emit on itself (for every nuki connected to bridge).
  
  @class Callback
  @param {Bridge}    connection        the bridge
  @param {Number}    callbackId        the id of the callback
  @param {Number}    callbackUrl       the url of the callback
  @param {Nuki}      [nuki]            the nuki
  @constructor