add_user(name, read_only=True) should update existing user in MongoDB 2.6, PYTHON-643.
Fix GridFS example code.
added failure cases for x509 auth tests
Fix tox compatibility.

The tox testing tool has dropped support for python 2.5
(and therefore jython) and by default sets PYTHONHASHSEED
to a random value. pip is dropping python 3.1 in its next
release so we might as well drop that from tox.ini now as
well.