ADD do-when-up.

Script to wait until a given host is online as determined by ping then
execute the given command.
Update README.md
  Renamed do-when-up to be when-up.

  This seems saner to me.
  Updated comments/usage.

 These now refer to the new name `when-up`, rather than `do-when-up`.
  Moved when-up section to correct place.

  Since it was renamed it now belongs in the `w`-section.
  Updated to use /bin/sh

  By dropping the fancy positional-argument extraction, and using
 `shift` we can run under dash, etc.