Add SPDX base tests.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
SpecVersion tag corrected to SPDXVersion in tag writer.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Adds Equal()bool methods to all SPDX Elements.

Adds SameLicence function to compare two AnyLicence.

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Add tag parse-write-parse test, close #45

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>