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 ("/") not the directory from which](https://github.com/rack/rack/blob/master/lib/rack/server.rb#L254)