Provides __str__ method for spdx.document.License

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Updates sdpx.package.Package docstring

Signed-off-by: Ahmed H. Ismail <ahm3d.hisham@gmail.com>
Completes implementation of package tag value writer method

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