code: function (method, handler) {
              if (!method || !handler) {
                  return;
              }
              if (!this.handlers[method]) {
                  //initialize array
                  this.handlers[method] = [];
              }
              // add handler to the stack
              this.handlers[method].push(handler);
          }
docs: Subscribe handlers to specific messages.
  @param {string} method Message type.
  @param {function} handler.
  TODO: add handler name or any identification mechanism to then implement 'off'?