Fix RedisSession.created.

Change RedisSession.created to represent the time the session was first
created, rather than the time this particular session *instance* was
initialised.
Re-arrange imports.
Remove support for Python 2.6.
Update some docstrings.

Update some docstrings that should have been updated following the
change from persisting only the session dict to persisting a tuple.
Make minor adjustments to imports.
Move timeout out of managed_dict, and serialise a dict instead of a tuple.

Move timeout out of managed_dict, as it is metadata that is not part of
the session. Previously any timeout set via
.adjust_timeout_for_session() would be lost after .clear(), for example.

Serialise session dict, created time, and timeout for storage in Redis
as a dict instead of a tuple, as a dict is more extensible, more
readable and less bug-prone.
Fix invalidate.

This fix adds support for the use of a new session within the same
request after an .invalidate() call. invalidate() now deletes the
session_id key from Redis, and RedisSession now knows to start a new
session with a new session_id when it is needed. (Previously
invalidate() only cleared the session dict and set a header to delete
the cookie, and did not take into account the use of the session after
invalidate().)

It also fixes the cookie setting after invalidate(). Previously
invalidate() just added another Set-Cookie header to the response to
delete the cookie, meaning there could be more than one Set-Cookie
headers for the same cookie name, which should not happen according to
RFC 6265. This has been fixed to set one and only one Set-Cookie header
(if any is needed) in the response callback, and to handle the various
combinations of whether the request started with a new session or
existing session, whether a new session was used after invalidate(),
more than one invalidate()s in the same request, and the
cookie_on_exception setting.
Small correction to docstring.

The signed secret being bad is just one of the possible reasons why the
value cannot be deserialized.
Remove RedisSession's timeout parameter and default_timeout attribute.

They are no longer needed now that the timeout is inserted into Redis by
the factory at the beginning of a new session.
Correct default value of ``cookie_httpOnly`` in docstring.

``cookie_httponly``'s default is True, but docstring had it as False.
Remove unnecessary signed_serialize parameter.

It was already accessible as an import.
Change asserts to use a different approach.

To assert that a header is one that sets a cookie as opposed to one that
deletes a cookie, we no longer assert the serialized session_id. This
means the tests would not break if `response.set_cookie()` makes any
changes to the serialized session_id before inserting it into the
cookie, as WebOb<=1.3 did by quoting certain characters.

The alternative approach chosen is to assert that 'Max-Age=0' is not in
the header string. A Set-Cookie header is either one to set cookie (in
which case 'Max-Age=0' would not be in the string), or one to delete
cookie (in which case 'Max-Age=0' would be in the string.)

It is a negative assertion, but all the alternatives would make the
tests too complicated and/or difficult to make sense of. This is the
least bad approach I can see for now.
Add tests for cookie-related factory parameters.
Rearrange ordering of cookie_ parameters.

Rearrange the ordering to be consistent with the factory's own
parameters and docstring, so that any missing parameters would be easier
to spot.
Fix bug where `cookie_path` parameter had no effect.

`cookie_path` parameter had no effect because it was not used by the
factory at all. We now set and delete cookie with its value as intended.
Fix bug with `cookie_domain`.

The factory was not making use of the `cookie_domain` value when setting
a header to delete cookie, meaning any cookie with a domain different
from the default would not have been deleted (as a cookie only gets
deleted if the path and domain match the ones used when the cookie was
set).
Update changelog.
Merge pull request #37 from whiteroses/fix.session_created

Fix for RedisSession.created