Draft asyncio API

1. Support basic data communications use cases with asyncio semantics, ie. awaitable operations

• sending WS-Discovery SOAPEnvelope message objects as defined in the generic base daemon
• sending WS-Discovery UDP messsage objects with support for unicast, multicast, retrying etc.
• receiving WS-Discovery SOAPEnvelope message objects as defined in the generic base daemon

2. Support integrating WS-Discovery into asyncio-based applications

Some ideas:

• callbacks for when a (particular type of) message is received (does not necessarily benefit asyncio-based apps specifically)
• awaitables for when a (particular type of) message is received