add --pid option

In order to send signals easily, add --pid option which stores process pid into file.
We can send signals like that:

```
$ ruby ./nadoka.rb -r ./nadokarc --pid ./pid/nadoka.pid
$ kill -s QUIT `cat ./pid/nadoka.pid`
```

This option is very useful when running nadoka with monit.
http://mmonit.com/monit/