Update for upstream IO changes.

`FileInfo` is gone, `std::rt::io` has been graduated to `std::io` and
file interactions are done via `std::io::File`.
Update the README example to be more-compilable.

I'm not really sure how rustpkg is supposed to work (and haven't got it
to work locally ever, because of bugs in it), so let's just leave the
detail of installing it/`extern mod`-ing it off for the moment.
Merge pull request #6 from huonw/io

Update for upstream IO changes.