Enable interrupts for received packets

• #33 send and receive methods modified to allow interrupts from user-code.

  • Example of usage with Raspberry Pi

• #33 send can now listen after a packet is sent using new kwarg keep_listening

  • Example of usage on Microcontrollers.

To use in CircuitPython, simply install the [Adafruit CircuitPython bundle] (https://github.com/adafruit/Adafruit_CircuitPython_Bundle).

To use in CPython, pip install adafruit-circuitpython-rfm9x.

Read the docs for info on how to use it.