Update AUTHORS file.

Following a private discussion among the project owners, we have decided to update the AUTHORS.rst file so that it reflects better the core contributors of the project (or past core contributors).

Since this is a sensitive subject, instead of completely deleting names, I've moved them to a new "earlier versions" section in the "what's new file". The contributors that I moved were confirmed by David to not have made substantial contributions.

Any suggestion / comment welcome.