Restart unicorn using USR2 (depends on unicorn.rb)

Monit ignores 'restart program = ... ' so we hack it using stop and a
fake start command. Unicorn USR2 is a rolling restart and it relies on
somebody to kill the old master and workers. Currently unicorn.rb is
configured correctly for this; the new unicorn will remove the old one.
Merge pull request #70 from dosire/unicorn_monit_start_stop

Restart unicorn using USR2 (depends on unicorn.rb)