Negative altitude values are not handled properly

[sba923]

Version 0.1.2 of the converter doesn't process negative values of the Altitude field in GPS data records properly.

The proper code (once Altitude is changed to a float, see #14) should be:

func parseAltitude(file *os.File, rec *Record) error {
	value, err := parseLong(file)
	if err != nil {
		return err
	}
	if value > 2147483648 {
		rec.Altitude = (float64(value) - 4294967296.0) / 10.0
	} else {
		rec.Altitude = float64(value) / 10.0
	}
	return nil
}