Fix Unix domain socket setting.

This code was misconceived in that the host parameter for a Unix
domain socket connection must point to the directory containing
the socket, and not to the socket itself. Libpq will look for
the socket based on the host and port settings. See
<http://www.postgresql.org/docs/current/static/libpq-connect.html#LIBPQ-PARAMKEYWORDS>
Merge pull request #281 from adunstan/unix-domain-socket

Fix Unix domain socket setting. closes #277