support for xiaomi aqara thermometer and few other fixes (#18)

* import pip._internal.main to work with pip version > 10

* make auth optional and add support for an optional port

* add worker for xiaomi aqara thermometer

* add support for optional global publishing topic_prefix

* Update config.yaml.example

mqtt.py

@@ -6,34 +6,53 @@ class MqttClient:
   def __init__(self, config):
     self._config = config
     self._mqttc = mqtt.Client(client_id='bt-mqtt-gateway', clean_session=False)
+    
+    if self.username is not None and self.password is not None:
+      self._auth = {'username': self.username, 'password': self.password}
+    else:
+      self._auth = None
 
   def publish(self, messages):
-    if messages is None:
+    if messages is None or len(messages) == 0:
       return
-
+    
+    if self.topic_prefix is not None:
+      for m in messages:
+        m.topic = "{}/{}".format(self.topic_prefix, m.topic)
+    
     publish.multiple(list(map(lambda m: m.as_dict, messages)),
                      hostname=self.hostname,
-                     auth={'username': self.username, 'password': self.password})
+                     port=self.port,
+                     auth=self._auth)
 
   @property
   def hostname(self):
     return self._config['host']
 
+  @property
+  def port(self):
+    return self._config['port'] if 'port' in self._config else 1883
+
   @property
   def username(self):
-    return self._config['username']
+    return self._config['username'] if 'username' in self._config else None
 
   @property
   def password(self):
-    return self._config['password']
+    return self._config['password'] if 'password' in self._config else None
+
+  @property
+  def topic_prefix(self):
+    return self._config['topic_prefix'] if 'topic_prefix' in self._config else None
 
   @property
   def mqttc(self):
     return self._mqttc
 
   def callbacks_subscription(self, callbacks):
-    self.mqttc.username_pw_set(self.username, self.password)
-    self.mqttc.connect(self.hostname)
+    if self.username is not None and self.password is not None:
+      self.mqttc.username_pw_set(self.username, self.password)
+    self.mqttc.connect(self.hostname, port=self.port)
 
     for topic, callback in callbacks:
       self.mqttc.message_callback_add(topic, callback)