Initialzie a new `SubEmitterSocket`.

@api private
function SubEmitterSocket() {
  this.sock = new SubSocket;
  this.sock.onmessage = this.onmessage.bind(this);
  this.bind = this.sock.bind.bind(this.sock);
  this.connect = this.sock.connect.bind(this.sock);
  this.close = this.sock.close.bind(this.sock);
  this.listeners = [];
}