Implements LicenseConjuction and LicenseDisjunction classes

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds File attributes to tag/value parser except artificat_of_*

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds LicenseLise parser and lexer

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Modifies package parser test case

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds docstring to License from_identifier and from_full_name methods

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds documentation to spdx.utils

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds docstring to Document.validate method

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Adds docstring to spdx.creationinfo.CreationInfo.validate

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
adds docstring to spdx.version.Version.from_str

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