Add MQTT to Library

[brentru]

This library currently only supports HTTP via a RESTClient class. With CircuitPython_MiniMQTT, we can now support a new class named mqtt to interface with the Adafruit IO MQTT API.

• Add MQTT class
  • Class should remain similar to Adafruit IO Python's MQTT Class.
• Add examples for using CircuitPython Adafruit IO with MQTT

MiniMQTT: https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT

[brentru]

Work on this issue is being performed on this fork's branch:
https://github.com/brentru/Adafruit_CircuitPython_AdafruitIO/commits/add-mqtt-api

[brentru]

Added in master and released. Closing...