Add Multiple Airplay2 Streams #436

At the moment only one Airplay2 stream can be running at a time, this is because you can effectively only have one instance of shairport-sync running Airplay2 per IP address. To get around this we'll have to make shairport-sync run in a docker container, getting audio out of a docker container, however, seems blocked until we're using a 64 bit OS.

For now if you try to start multiple Airplay2 streams one will say "An Airplay2 stream already exists! Please disconnect it and try again."