Postgres defaults to unix socket with no host #229

Closes #229

When there's no host given in a postgres connection url, use the unix
domain default socket. Previously, the default was a localhost TCP
connection. This default behavior is more in line with standard postgres
clients out there, like psql.

pkg/driver/postgres/postgres.go

@@ -48,7 +48,7 @@ func connectionString(u *url.URL) string {
 
 	// default hostname
 	if hostname == "" {
-		hostname = "localhost"
+		query.Set("host", "/var/run/postgresql")
 	}
 
 	// host param overrides url hostname