On Windows, FileUtils.cp(path, dest_path) will fail with a Permission
Denied if the dest_path already exists and is read-only. People have
complained about this since at least 2008.
This lets `jekyll build` work without error in Windows when one or more
read-only files exist.
This change will accommodate users of Team Foundation, which uses the
read-only flag for for source control status. (dumb, I know, but but a
real problem)
Also make sure static files get regenerated when they are missing in destination.
This is useful in --server --auto mode when it reduces disk/cpu load and also plays nice with xrefresh-server (which was my main motivation) -> soft CSS refresh works again!