[referencer] Corrected and enhanced genkey.py

[Renaud Lacour]

Hello,

First of all, thanks for the previous post which enabled me to discover
genkey.py.

I propose a small correction (mis-split when an author's name contains
"and" as a substring) and the handling of the three following marks:

%sa = first three letters of the first author's surname
%A = all authors' surnames
%sA = first three letters of authors' surnames