Padrino::Server - call expand_path on PID file option

[sshaw]

Normally Rack::Server would expand the PID (-P) argument. 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 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:

> 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