Option to chdir once clockworkd starts

Daemons runs `Dir.chdir('/')` when it forks each process. This causes problems for anything that expects `Dir.cwd` to be the rails root directory.

So, it'd be great if you could pass an option to clockworkd, something like `--dir=/srv/www/app/current`.

Admittedly, this is arguably something that daemons should solve, but it's a problem I came across attempting to run clockwork.