Add the Hoa\Socket\Server\Group for semantics.

“Wrapper” around Hoa\Socket\Server\Handler::merge() to avoid
ambiguities and add code semantics.
Example:

    $websocketA = new Hoa\Websocket\Server(
        new Hoa\Socket\Client('tcp://127.0.0.1:8880')
    );
    $websocketA->on('message', …);

    $websocketB = new Hoa\Websocket\Server(
        new Hoa\Socket\Client('tcp://127.0.0.1:8881')
    );
    $websocketB->on('message', …);

    $websocketC = new Hoa\Websocket\Server(
        new Hoa\Socket\Client('tcp://127.0.0.1:8882')
    );
    $websocketC->on('message', …);

    $group   = new Hoa\Socket\Server\Group();
    $group[] = $websocketA;
    $group[] = $websocketB;
    $group[] = $websocketC;

    $group->run();