Changes
* Added timestamp support (@mariam-japaridze)
* Added headers support (@martynaszaliaduonis, @dariuskasiulevicius)
* Added Rdkafka\Conf::setConsumeCb(), RdKafka\Conf::setOffsetCommitCb() (@tPl0ch)
* Added RdKafka\KafkaConsumer::getCommittedOffsets() (@dariuskasiulevicius)
* Fixed RdKafka\Message::errstr() (@JustBlackBird)
* Fixed reflection (@carusogabriel)
* Allow null key and null message (@awons)
* Dropped official PHP 5.4 / 5.5 support (@tPl0ch)
* Improved examples (@dbakiu, @Steveb-p)