tag package refactoring.

pair struct is now unexported
parse(io.Reader) ([]pair, error) is now unexported
parseDocument([]pair) (*spdx.Document, error) renamed to build

Signed-off-by: Vlad Velici <vlad.velici@gmail.com>
Add tag/api.go - the exported functions of tag package

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