Enable sslmode for PostgreSQL connections

By default, PHP 5.5 and PostgreSQL PDO will not allow SSL connections. This adds the default of "allow", but also the user may override the setting to "require" or any of the other potential values. 

See: http://php.net/manual/en/function.pg-connect.php
Update database.php.default to include sslmode
Add sslmode support for postgres.

While 2.6 is in RC this pull request was open well before that, and
I simply forgot to merge it in.

Refs #5173