Enable interrupts for received packets

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

  • Example of usage with Raspberry Pi

• #19 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.

To use in CPython, pip install adafruit-circuitpython-rfm69.

Read the docs for info on how to use it.