Add Topic-specific message callbacks

[brentru]

Addresses issue: #37

• Adds add_topic_callback: Registers a callback_method for a specific MQTT topic.
• Adds remove_topic_callback: Removes a registered callback method
• Setting on_message = callback_method from user_code is performed using a getter/setter.
• MQTTMatcher, MQTT topic format matcher added from Eclipse Paho MQTT https://github.com/eclipse/paho.mqtt.python/blob/master/src/paho/mqtt/matcher.py
• Licensing updated to reflect new code.

---

Tested using: examples/minimqtt_pub_sub_blocking_topic_callbacks.py
Tested on: Adafruit CircuitPython 5.3.0 on 2020-04-29; Adafruit Feather Bluefruit Sense with nRF52840 with AirLift FeatherWing

REPL output

code.py output:
Connecting to WiFi...
Connected!
Connecting to MQTT broker...
Connected to MQTT Broker!
Subscribed to device/# with QOS level 1
Battery level: 3.5v
New message on topic device/sensor1: 3.5
New message on topic device/sensor1: 500

Mosquitto Publisher Client

mosquitto_pub -h IP -p PORT -m 3.5  -t device/batteryLevel
mosquitto_pub -h IP -p PORT -m 500  -t device/sensor1

Test for remove_topic_callback, modified minimqtt_pub_sub_blocking_topic_callbacks.py

def on_battery_msg(client, topic, message):
    # Method called when device/batteryLife has a new value
    print("Battery level: {}v".format(message))

    # Remove this topic callback
    client.remove_topic_callback("device/batteryLevel")

Mosquitto Publisher Client

mosquitto_pub -h IP -p PORT -m 3.5  -t device/batteryLevel
mosquitto_pub -h IP -p PORT -m 3.5  -t device/batteryLevel

REPL output

code.py output:
Connecting to WiFi...
Failed to connect, retrying
 ('Failed to connect to ssid', b'TP-Link_519E')
Connected!
Connecting to MQTT broker...
Connected to MQTT Broker!
Subscribed to device/# with QOS level 1
New message on topic device/sensor1: 500
Battery level: 3.5v
< remove_topic_callback executed here >
New message on topic device/batteryLevel: 3.5 <<< topic filters to on_message instead 

[ladyada]

@jimbobbennett hi do ya wanna review this one?

[jimbobbennett]

@jimbobbennett hi do ya wanna review this one?

Yup - I'll take a look next week and test it out by upgrading the Azure IoT library to make use of it.

[jimbobbennett]

Looks good now. Tested with the Azure IoT library.

PR raised for the Azure library to update to use these changes once merged.

[brentru]

Perfect - thank you for testing it against that library.