Add support for sslcert and sslkey

This commit adds support for both automatically loading certificates
from ~/.postgresql/ and loading them from arbitrary paths by using
sslcert and sslkey (or their environmental equivalents PGSSLCERT and
PGSSLKEY).  If their paths are set explicitly, the files must exist or
connection attempts will fail.  The behaviour was designed to exactly
match that implemented by libpq.

Also add some basic testing for SSL features into Travis.  Previously
the SSL support was not tested at all.  The tests are not run unless
some special environment variables are set, since we can't run these
tests without special support.

James Pozdena (@jpoz) and Marko Tiikkaja (@johto).