Add option for relaying all messages upstream.

As a sort of primitive demo mode, this lets you try failmail without
preventing the normal flow of error emails. Though, this is possibly not
great, because we're still running the original email through quite a
bit of processing before it gets sent back up.
Add support for upstream AUTH PLAIN.

For now, it'll automatically use STARTTLS if it's supported, so this is
enough to use e.g. Gmail as a relay.
Add an example Python configuration.
Add an example Java (log4j) configuration.