Propagate return values of Paho's original client methods

[flbraun]

Propagate return values of Paho's original Client.publish and Client.subscribe methods back to the caller of MqttClient. This allows writing code on MqttUser instances as if one was dealing with the original client, e.g.

class MyUser(MqttUser):
    @task
    def report_battery_level(self):
        msginfo = self.client.publish('reporting/battery', b'soc=99')
        msginfo.wait_for_publish()

[cyberw]

Nice.