Fixes type on spdx/creationinfo.py

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds to_value functions to Creator classes

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds has_comment method to Review, Document and Package models.
Also Implements tag/value writer except for package and extracted_licenses methods which are stubs.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds tag/value writer example file

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Updates readme with new example and status

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Reformats spdx/writers/tagvalue.py

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