Allow log_dir option to be given to Daemons.run

Utilizes the `:log_dir` option of the [Daemons](http://daemons.rubyforge.org/classes/Daemons.html) gem to allow users to place their pid files in a separate directory than their log files.

For example, you can now launch scheduler_daemon as such:

`bundle exec scheduler_daemon start -- --pid_dir=tmp/pids --log_dir=log/`