make url_to_dsn smarter

[chadwhitacre]

Postgres >= 9.2 can parse URLs itself, so we should check versions and pass through.

We should also be smart about not including host in the connection string at all if it's not set in the URL, because that affects whether Postgres uses a Unix or TCP socket. See https://botbot.me/freenode/gittip/msg/10532161/.

[zbynekwinkler]

From http://www.postgresql.org/docs/9.3/static/libpq-connect.html

The host component is interpreted as described for the parameter host. In particular, a Unix-domain socket connection is chosen if the host part is either empty or starts with a slash, otherwise a TCP/IP connection is initiated.

[chadwhitacre]

Blech. We can't easily version-sniff, because we don't have a connection to the database yet at this point (of course).

Complicating matters is the fact that Python's urlparse behaves differently than Postgres:

psql postgres://foo@/bar sees user=foo, but Python sees foo@ as the netloc. Ick.

[chadwhitacre]

What we should do is try it as an URL and if we get OperationalError then parse to a DSN and retry.