Refactoring of the discovery feature

[fabaff]

• Move discovery to its own file
• Replace global FOUND_BULB_IPS with a poor-man's "bulb registry"
• Allow to set the broadcast address (fixes ##28)

import asyncio
import logging 
from pywizlight.discovery import find_wizlights

async def main():
    bulbs = await find_wizlights()
    for bulb in bulbs:
        print(bulb.__dict__)

if __name__ == "__main__":
    logging.basicConfig(level=logging.DEBUG)
    loop = asyncio.get_event_loop()
    loop.run_until_complete(main())

Broadcast address can be set:

bulbs = await find_wizlights(broadcast_address="255.255.255.255")

This should be tested, please.