WIP authors: initial release

* Initial module structure.
* Introduces SQLAlchemy models for Publication, Author and Signature.
* 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>
Please make your code Python 3 friendly:
`except MultipleResultsFound, e:` -> `except MultipleResultsFound as e:`

run `pep8` and `pep257` in your module. You should fix everything **except** `##` in licence header (ongoing RFC #1904).