Add support for connecting through Unix sockets

Includes connection test (assumes socket is in /tmp, the default location).
test_unix_connection now detects the socket directory.

Change pg_hba.conf to allow connections through the socket.

Ignore connect_unix doc test. It requires `extern crate url;` which is not allowed with rustdoc.

Also, per comments on PR #35:
- Inline open_unix
- Centralize common code from connect and connect_unix in connect_finish.
Merge pull request #35 from zr40/add-unix-domain-socket

Add support for connecting through Unix sockets