[IMP] mail, email_template: mass mailing feature cleaning

[IMP] mail, email_template: mass mailing feature cleaning
- now differentiates mass mailing and mass post.
- mass mailing is a true mass mailing
- using same_thread log a copy in the document, without using message_post
(using notification field, + model and res_id)
- improved form view of composer, adding a filed explaining a bit the various
recipients
- removed unnecessary fields coming from the template (partner_to, ...) because
they are confusing -> composer should be easier to understand and use
- removed some unnecessary code
- removed double body computation when using templates (one for tmeplate, then
the wizard -> not necessary)

This commit will be followed by other to try to improve the mass mailing
and mass post.

bzr revid: tde@openerp.com-20140220152945-ash0hfkzevzamihq