Fix copy when force=no and update _remote_md5 docs.

I noticed today that when I set force=no on the copy module then the file would not be copied to the destination even if it did not exist. After testing return codes from a few flavors (Mac, Ubuntu, CentOS) of md5 it turns out '1' is returned when a file doesn't exist; not sure about return codes of other flavors.

Previously setting force=no caused copy to subversively
fail when target did not exist on remote host.

Caused by Runner._remote_md5 returning 1
when files don't exist, rather than 0.