code: function Event (meta, evtFn) {
    Definition.call(this, meta);
  
    meta = meta || {};
  
    if (evtFn && !_.isFunction(evtFn)) {
      var err = new Error('Event function not injected!');
      debug(err);
      throw err;
    }
  
    this.version = meta.version || 0;
    this.payload = meta.payload === '' ? meta.payload : (meta.payload || null);
  
    this.evtFn = evtFn;
  }
docs: Event constructor
  @param {Object}   meta  Meta infos like: { name: 'name', version: 1, payload: 'some.path' }
  @param {Function} evtFn Fuction handle
  `function(evtData, aggregateModel){}`
  @constructor