Python3 cffi bindings

[Floris Bruynooghe]

Hi all,

IIRC there was a thread in August about another attempt at bringing
the CFFI-based bindings on board as a Python3-only version.  I
believe there was a desire to re-name things but my searching-fu is
failing me and I can no longer find the email thread.

Anyway, I found the code, checked things work, updated tests on new
python versions, added a very basic intergration with the test
framework and squashed the commits.  Otherwise the attached patch
is just a plain dump of the current state so interested people have
at least a copy of the code again which can be made to work.

IIRC this probably wants to be renamed to "notmuch2" instead of
"notdb".  Otherwise I'm pretty sure this doesn't cover all the
current features either.

So maybe this can be used as a start to figure out how to merge
this if there's still an interest in this.

Cheers,
Floris