actual mock objects from the mock library.  I'd like to use mock
for new tests so we might as well use it in obvious places.
- use unittest.mock in py3.3
- changelog
- add a note to README.unittests
- add tests_require in setup.py
- have tests import from sqlalchemy.testing.mock
- apply usage of mock to one of the event tests.  we can be using
this approach all over the place.

Postgresql dialect to no longer inject a hardcoded ``::timestamp``
or similar cast into the given expression, as this interfered
with types such as timezone-aware datetimes, but also
does not appear to be at all necessary with modern versions
of psycopg2.  Also in 0.8.2.
[ticket:2740]