Don't redirect stdout/stderr when BACKGROUND=true

Ruby 1.9's Process.daemon redirects stderr/stdout to /dev/null
which makes logging impossible. This change prevents
Process.daemon from redirecting stderr and stout to /dev/null
unless ENV['MUTE'] is set.
Merge pull request #156 from nmeans/c45d9977bc1448d34fa09c94b4516e950bac0d70

Keep BACKGROUND from automatically redirecting stderr/stdout to /dev/null