Add a way to connect through UNIX socket.

[apognu]

This PR adds the ability to instruct Postgres to listen on UNIX sockets instead of on a TCP port.

It adds a WithoutTcp() method to the configuration to make the switch. By default, it follows Postgres's default to creating the socket in /tmp, but the socket path can be customized through WithUnixSocketDirectory(dir string).

It modifies the config's GetConnectionURL() to take the current mode of connection into account, and adds a public GetConnectionURL() method on EmbeddedPostgres to proxy it (I would have preferred for those to return *url.URLs, but did not want to change the signature of public methods.