Add Linux support.

Hi,

Mackup is a better solution than my lashed up shell script, but it didn't speak Linux, so I've added initial Linux support. Mackup now uses `platform.system()` to use...

  *  `setfacl` instead of `chmod`
  *  `chattr` instead on `chflags`

...when Linux is detected. Original Darwin behaviour is unchanged.

Regards, Martin.