Synchronize contacts across a set of GMail accounts.  The accounts may be normal
GMail accounts or in Google Apps.  The script is tested for synchronizing two
users and only marginally tested for three or more.

Unlike most shared group synchronizers, this script synchronizes all group names
and then all contacts except for contacts in groups named --private on the
command line.

This contacts-sync Python script depends on the Google Data APIs Python Client
Library (https://code.google.com/p/gdata-python-client) which must be installed
separately.

IF YOU READ NOTHING ELSE:  Pay particular attention to the way contacts are
merged on the first run.  Duplicate contacts across users will result in
duplication everywhere.  See the synchronize all contacts section below.

Accounts and passwords are stored in ~/.google/config.  See the sample/config
file here for an example.

The algorithm:

 - Synchronize all group names:

   - When first seen, groups are tagged with a unique identifier (UID).
   - Equivalent groups in each user's account have the same UID.
   - For new groups without a UID, look for a group of the same name in one of
     the other user's accounts.
   - If a group has been renamed by a user, propagate the name change to
     all groups sharing the UID.

 - Synchronize all contacts:

   - When first seen, contacts are tagged with a unique identifier (UID).
   - Newly seen contacts are replicated across all groups unless:
         They are not in any group or
         they are in a private group.

     NOTE:  There is no effort made to match new contact names to give
            them a shared UID.  Replication will cause each instance to
            be replicated everywhere.  If you then use the Google tools
            to merge contacts in one account, the next synchronization
            will replicate the merge everywhere.

            Avoid this when adding a new account with the --addingnewaccount
            switch, which attempts to find equivalent names and assign
            existing UIDs to them.  Differences in contact data for
            existing names may still cause problems.

   - Once tagged with a UID, contacts are linked by UID and not by name.
     Renaming a contact in one account will replicate properly to other users.
   - Modified contacts replace their corresponding entries in all other
     accounts.  The script does not detect modification of the same entry
     in multiple accounts.  One of the modified entries will be replicated
     and changes to the same contact in other accounts will be lost.
   - If a contact was replicated and has been newly added to a private group,
     replicated copies in other user's accounts are deleted.
   - If a replicated contact is deleted in one account it is deleted in
     all accounts.