Psycopg: the green branch

Hello,

albeit I don't know if the coroutine support will be released in
psycopg 2.2.0, the branch is now finished and can be used for testing.

The code can be obtained from the "green" branch in my git repos
(https://www.develer.com/gitweb/pub?p=users/piro/psycopg2.git;a=shortlog;h=refs/heads/green)
and is based on what's current in the python2 branch, including Jan's
async support.

I've created a small project
(http://bitbucket.org/dvarrazzo/psycogreen/) where there is
integration and test of psycopg with the Eventlet and gevent
libraries, with the idea of having such support integrated in the
"greening" procedure of the above libraries.

The Psycopg test suite can be run with the external wait callback
using something like:

    PSYCOPG2_TEST_GREEN=1 make check

all the tests in the suite pass in "green" mode.

Currently copy is still a blocking operation, but it can be made green too.

Comments and ideas are welcome.

-- Daniele