Package docstring

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Modifies tag/value parser to handle NONE value as utils.SPXNone

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Fixes bug in package.validate_str_fields.
Implements package.validate_checksum.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Updates spdx.file.File docstring.
Adds add_lics, add_contrib, add_depend and add_artificat methods to spdx.file.File class.
Implements spdx.file.File validate method.

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds add_lics_from_file to spdx.package.Package

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Implements write_file method in spdx.writers.tagvalue module

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Implements validate_file_lics_in_file method in spdx.parsers.validations module

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds file and package information to write tag/value example file

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Changes spdx.parser.tagvaluebuilders.FileBuilder
to correctly set checksums as spdx.checksum.Algorithms instances.

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