Support mDNS to resolve the server

[deanward81]

When using the CLI we currently need to specify the server/port to connect to:

AirDropAnywhere client --server localhost --port 8181 --path ~/Downloads

By using mDNS we can resolve the hostname and port by advertising a service (e.g. _airdrop_proxy._tcp.local) that we can lookup at runtime.

Ideally we'd be able to do this in a web browser as well, but SRV records aren't supported. For now web browsers will need to use https://server:port/ :(