function AlertMessage(arg, options) {
  Message.call(this, options);
  this.command = 'alert';
  if (!arg) {
    arg = {};
  }
  this.payload = arg.payload || new Buffer(32);
  this.signature = arg.signature || new Buffer(32);
}
@param {Object=} arg
@param {Buffer=} arg.payload
@param {Buffer=} arg.signature
@param {Object} options
@extends Message
@constructor