authors: initial release

* Initial module structure.

* Introduces SQLAlchemy models for Publication, Author, Signature and
  Citation.

* Introduces claiming operations on signatures.

* Adds citation association table.

* Tests for models.

Signed-off-by: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Co-authored-by: Gilles Louppe <gilles.louppe@cern.ch>
Co-authored-by: Mateusz Susik <mateusz.susik@cern.ch>
Co-authored-by: Evangelos Tzemis <evangelos.tzemis@cern.ch>