attempts when an existing connection attempt is blocking.  Previously,
the production of new connections was serialized within the block
that monitored overflow; the overflow counter is now altered within
it's own critical section outside of the connection process itself.
[ticket:2880]

connection to be available, such that for a connection pool
with no timeout specified, it will every half a second break out of
the wait to check for the so-called "abort" flag, which allows the
waiter to break out in case the whole connection pool was dumped;
normally the waiter should break out due to a notify_all() but it's
possible this notify_all() is missed in very slim cases.
This is an extension of logic first introduced in 0.8.0, and the
issue has only been observed occasionally in stress tests.

- use thread.join() for waiters_handled test

Conflicts:
	test/engine/test_pool.py

_ConnectionRecord.checkin() so that this functionality is
encapsulated; is now called from two different locations
for [ticket:2772].
- Fixed bug where :class:`.QueuePool` would lose the correct
checked out count if an existing pooled connection failed to reconnect
after an invalidate or recycle event. [ticket:2772]

at the nose command line

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.