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>
You can use `.format(signature=signature, author=author)`:
```python
raise SignatureExistsError(
    "Cannot reassign signature {signature} to "
    "author {author} for publication {signature.publication}. The author already has a "
    "signature for this publication.".format(signature=signature, author=author))
```