Comments and code organisation for tag/builder.go

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Backing up before making a massive logic change to uncommited code

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Finished parsing []Pair returned by tag.Parse to spdx.Document logic.
close #1

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