:groupby: for sphinx bib directives + improvement of __doc__

This one does need some code polish and making it less ad-hoc but not sure if I would have time... seems to work though.  It has potential to break things -- so would be nice if you could try/review.

But otherwise -- it should be very handy to automatically provide reasonably structured bibliographies.  See e.g.

http://haxbylab.dartmouth.edu/publications.html

which was generated from a single bibmatching entry with groupby