Facilitate PublishOnly uses

[Jeppedy]

When I'm using a "publish-only" device, no QoS, I don't believe I actually need to register a callback function. But, there's no signature to allow for that.
It's not a big deal, but it's not clean...
Can you make a constructor that does not require the callback, and will throw an exception if subscribe() is called...

[hirotakaster]

Simple publish-only case like this, please use callback-function is NULL.
MQTT client("MQTT server", 1883, NULL);