Support for multiple users/accounts

Currently, Spamato is restricted to a single user. All
messages are handled as if they have been sent to a
single user or account. Even the Spamatoxy, that allows
access even from several users, "shares" one instance
of Spamato. This is only sub-optimal as users (and
event different accounts of one user) might differ
significantly from each other.

To allow different users to use a single Spamato
instance, we have to enable the plugin system to
support multiple users. All plugins must access their
data from a new "user-aware session object".
Additionally, plugins must take into account this new
behavior---they, cannot store global variables or state
information outside the session object.

Here are some additional ideas just to log them for
future consideration:

Multi-user/account support should be optional. For
easier handling, a user should also be able to choose
the old way. That is, for instance, if accounts aren't
that different, the user might want to have a single
session or rather to share knowledge among account
session as before. This would also ease administration
if we want to run a Spamatoxy for several users with
the same settings.

There will be several problems to be solved: For
instance: How to configure such a system? Using the
web_config with several users, we might have to add a
'login' since users shouldn't be able to change other
users' settings. How to distinguish between multiple
users and multiple accounts for a single user? How to
share sessions/data between users/accounts? And how to
configure all this?

To allow all this, it would also be necessary to update
the add-ons. A user with multiple accounts using
Thunderbird or Outlook should be able to modify its
settings for different accounts. But it shouldn't be
more difficult than with one account.

We can probably have a special implementation for
multiple users and multiple accounts for a single user.
We might also want to have different implementations
for the Spamatoxy (which might be used by several
users) and the other add-ons (S4O, S4T, S4M). We might
even end up with an integration version for mail servers.