Fix for RedisSession.created

The three commits:

##### Change `RedisSession.created` to represent the time the session was first created, rather than the time this particular session *instance* was initialised.
The tuple approach follows the one in Pyramid (https://github.com/Pylons/pyramid/blob/master/pyramid/session.py#L368), and keeps the value out of the session dict. If you think that's okay, I think it would also make sense to follow the same approach with the timeout value?

##### Re-arrange imports as we discussed.
PEP 8 mostly, and in test-related code place less dependable imports in test methods wherever possible.

##### Remove support for Python 2.6 from setup.py as we discussed.
Do you also want to add support for Python 3.4? I think there's a bug with Nose that breaks something with Python 3.4 at the moment, but the tests pass with `python -m unittest`.

Please let me know if there's anything you need me to change. Thanks!