OSX packages to test

Hello,

I've had some reverse tunnel fun to get into travis OSX machine and
use them interactively, and I have managed to create OSX binary
packages for psycopg. Like the manylinux version they contain a copy
of the libpq and the other libraries needed so it should be possible
to install them on machines where there is no compiler, python and
libpq header files, postgres client library installed.

Packages are provided for Python versions 2.7, 3.4, 3.5, 3.6 as I've
not been able to install other Python versions on Travis
(https://github.com/psycopg/psycopg2-wheels/commit/1cc87601) but I
guess that should be enough.

It would be great if you could test them, using:

pip install -i https://testpypi.python.org/pypi psycopg2==2.7b1

It has been reported
(https://github.com/psycopg/psycopg2/issues/385#issuecomment-279212629)
that installing wheels requires an up to date pip version, so a "pip
install --upgrade pip" could be helpful in case of problems.

Let me know if they work ok for you. Thank you very much.

-- Daniele