Padrino::Server - call expand_path on PID file option

Normally [Rack::Server would expand the PID (-P) argument](https://github.com/rack/rack/blob/master/lib/rack/server.rb#L184). But with `Padrino::Server`, there is no `ARGV` for `Rack::Server` to parse, so a PID option that's not absolute will cause `Padrino::Server` to silently fail when it daemonizes as it will be [attempting to open the file from the root directory ("/")](https://github.com/rack/rack/blob/master/lib/rack/server.rb#L254)  not the directory from which it was invoked. 

On a related note, why not just mimic the `Rack::Server`/`rackup` options? While it's is a minor issue, standardization is good. For example, this is somewhat confusing:

```bash
> padrino start -di some/pid/file.pid
> padrino stop -i some/pid/file.pid
padrino stop requires at least 0 argument: "padrino stop".
> padrino stop -p some/pid/file.pid
=> Sending INT to process with pid 57493 wait
```