Allow to use fastcgi via standard io.

Passing `nil` as the first parameter to `fcgi.Serve()` (e.g. `fcgi.Serve(nil, app.Handlers)`) will enable using standard I/O instead of tcp/unix sockets. This is useful to deploy applications in shared hosting environments where you are not allowed to use tcp/unix sockets.

I put together an example using go + apache + fastcgi via standard I/O here: https://github.com/bsingr/golang-apache-fastcgi.