Redeploy apps on reboot, fixes #82

It's a different approach from https://github.com/progrium/dokku/pull/88 and https://github.com/progrium/dokku/pull/94, but it's shorter, and we hope to remove it in the future (see https://github.com/progrium/dokku/issues/82#issuecomment-22781098). A possible + of this approach is that other plugins also get to run the pre- and post-deploy steps.