Support %r in ProxyCommand configuration in ssh_config files as defined in OpenSSH.

This is a fix of #25 in github, and #38 in lighthouse.
http://net-ssh.lighthouseapp.com/projects/36253/tickets/38-netssh-doesnt-work-if-a-proxy-takes-a-r

This commit changes the API of Net::SSH::Proxy#open().  Although this commit also let subclasses of Net::SSH::Proxy follow the change, please let me know if you have any other concern.