authors: base record

* Defines fields and models for author base record.
* Adds new function for jsonalchemy models definitions.
* Includes tests for author base record.

There is one thing to add here: We need to store the list of the publications. This is a relaton between an author and a publication and this directly touches the subject of monday's meeting.
`AUTHORS_IDENTIFIERS_TYPES` vs. `AUTHORS_IDENTIFIER_TYPES` ?