Zombie/Defunct process when monitoring using pidfiles

Looks like the `fork` process is still sticking around leading to a zombie process, if the main ruby thread is still running. 
https://github.com/thuehlinger/daemons/blob/master/lib/daemons/monitor.rb#L62-L63

Not that big of a deal but it could used up the process table.

Possible fix? 
```ruby
pid_fork = fork { ... }
Process.waitpid(pid_fork)
```