Creates spdx.utils module

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Creates Review information model.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Changes Creator to use utils module

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds reviews field to Document model

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>