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.

- Fixed potential memory leak which could occur if an
arbitrary number of :class:`.sessionmaker` objects
were created.   The anonymous subclass created by
the sessionmaker, when dereferenced, would not be garbage
collected due to remaining class-level references from the
event package.  This issue also applies to any custom system
that made use of ad-hoc subclasses in conjunction with
an event dispatcher.  Also in 0.7.10. [ticket:2650]

become an externally usable package but still remains within the main sqlalchemy parent package.
in this system, we use kind of an ugly hack to get the noseplugin imported outside of the
"sqlalchemy" package, while still making it available within sqlalchemy for usage by
third party libraries.

    fires off for so-called "_cursor_execute"
    events, which are usually special-case
   executions of primary-key bound sequences
    and default-generation SQL
    phrases that invoke separately when RETURNING
    is not used with INSERT.  [ticket:2459]

usage of the event system; instance-level
collections are no longer created for a
particular type of event until
instance-level listeners are established
for that event.  [ticket:2516] Also in 0.7.9.

which would primarily show up as
events not being registered with
sessionmaker() instances created
after the event was associated
with the Session class.  [ticket:2424]

target + event name, applies the decorated
function as a listener.  [ticket:2106]
- remove usage of globals from test.base.test_events