Clarify IPlugSession interface
Fix missing word
Add clear() function to IPlugSession
Add clear() function required by new interface
Clean up after ourselves, remove old sessions

If a session expires, there is no reason for the backend to hold on to
it, a new session needs to be created.
Add more cases where we regenerate the session id

In an attempt to stop session fixation attacks, we want to make sure we
don't reuse the same session ID after it has expired, or the backend
data failed to to unpack/deserialize.
invalidate() allows the session to be used again

After a call to invalidate() the session should be cleared, but ready
for use. At this point we reset the created timestamp, set the session
to new again, and generate a new session id.

The plug is told to clear the old session information first, so that in
the case of a session fixation attack the old session can not be reused.
Update CHANGES
Prepare for 0.0.0a2