User switching with `script/delayed_job`

I've added options `-u` and `-g` which are used to perform permissions changes. This prevents having to jump through hoops in production when starting the daemon with something like `upstart`.

I noticed another pull request (#266) open for this, but I didn't like that it duplicated existing behavior in the `daemons` gem. I also fixed a Rails 3 deprecation warning in a different commit (grays@da07875e), I didn't include it in the pull request because I don't want to make assumptions about why it wasn't already changed.