zeroconf extension

discover and advertise repositories on the local network

The zeroconf extension will advertise `hg serve` instances over DNS-SD so that they can be discovered using the `hg paths` command without knowing the server's IP address.

To allow other people to discover your repository using run `hg serve` in your repository:

$ cd test
$ hg serve

You can discover Zeroconf-enabled repositories by running `hg paths`:

$ hg paths
zc-test = http://example.com:8000/test