Add ignored() context manager

@raymondh added `contextlib.ignored()` to Python 3.4 in
http://hg.python.org/cpython/rev/406b47c64480. I think it's awesome and
I want to use it in Simon. Because 3.4 hasn't been released yet, no
stable release of Python has it. Plus, Simon currently only supports
Python 2.6 and 2.7, so there's no guarantee Simon will be able to use
the version from `contextlib` once it's available.

All of the code in this commit was written by @raymondh. He kindly
shared it in his "Transforming Code into Beautiful, Idiomatic Python"
talk at PyCon 2013:
http://pyvideo.org/video/1780/transforming-code-into-beautiful-idiomatic-pytho
Remove integration tests for PyMongo 2.2 in tox

Now that tox uses the most recent release of each PyMongo release, the
integration tests are getting hung up for PyMongo 2.2. This is the same
as the behavior that was noted for PyMongo 2.3 in b679d6a.

It would appear that this is the result of a call to `getLastError()`.
It was fixed in mongo-python-driver/a33b0fc. More information can be
found in JIRA ticket PYTHON-371:
https://jira.mongodb.org/browse/PYTHON-371