Adds possibility to change print for a callback function on messages in class Scanner

[sanyi]

Added output_callback init argument to class Scanner. So one can divert the data received to an external function rather than print

Ussage:

from queue import Queue
from victron_ble.scanner import Scanner

loop = asyncio.get_event_loop()
messages = Queue()

device_keys = (("DE:38:XX:XX:XX:XX", "somesecret"),)

async def scan(keys):
    scanner = Scanner(keys, output_callback=messages.put)
    await scanner.start()

asyncio.ensure_future(scan({k: v for k, v in device_keys}))
loop.run_forever()