Make the handling of driver-only settings more consistent

There seemed to be two different ways these were handled.  Pick one of
them and embrace it.
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).
Add James Pozdena to the list of contributors
Calculate the distro codename for APT
Add support for sslrootcert option and PGSSLROOTCERT env. variable
Fix documentation for sslcert / sslkey

All of the parameters are optional, so there's no need to mention it
separately for these two.
Add Charlie Melbye to the list of contributors
Add support for sslmode=verify-ca

This is like verify-full, except the Common Name isn't matched against
the "host" parameter.
Move environment variables to the env configuration