[IMP] mail.group: cleaned model. Now only have linked user groups, that...

[IMP] mail.group: cleaned model. Now only have linked user groups, that automatically subscribe its users to the mail.group instead of broadcasting or anything else. Removed old fields, cleaned view. Added code for res.users and res.groups to handle user changes in groups, propagating them to the mail.group attached to the res.group.

bzr revid: tde@openerp.com-20120719120026-fi5o23cnh17kapgu