Improve the auto-creation of the "bucardo" role.

When adding a database, that is. The old code led to confusing errors, where
the user would be told that Bucardo failed to connect as user "postgres" when
you had not, in fact, asked it to. So log more informat as it goes along,
noting failure to connect as "bucardo", trying to connect as "postgres", and
showing success or failure at doing so. This makes things much clearer to the
user what's going on -- and is less fussy, to boot.

Resolves #63.