Added support for post-db-connect callback.

If you put app.listen() for express apps inside the callback, it fixes many session TTL timeout issues that occur for many people.

Similar issues have been raised in connect-mongo, but have only been resolved through callbacks such as this. Since this library supports replica-sets like I'm using, I chose it, but suffered from session TTL issues before implementing this callback. Figured I'd share. :)

Thoughts? I'm super curious why this wasn't added in the beginning!