Set clockwork process name

Before:
```
$ clockwork config/clock.rb &
$ ps | grep clock
73872 ttys005    0:00.32 ruby /Users/fengb/.gem/ruby/2.1.2/bin/clockwork config/clock.rb
```

After:
```
$ clockwork config/clock.rb &
$ ps | grep clock
73949 ttys005    0:00.31 clockwork config/clock.rb
```