Check if nice or ionice commands exist

Some POSIX systems do not have nice or ionice support, notably Synology NAS boxes. Currently extraction and transcoding breaks on these systems: ``Could not call command ['nice', '-n10', 'ionice', '-c2', '-n4', 'unrar', 'x', '-o+', '-y', u'/path/to/rar', '-p-']`` even though unrar exists within the PATH.