[FIX] mail: when generating pure email mass mailing

[FIX] mail: when generating pure email mass mailing
we have to process attachments as well as attachment_ids in the values generated
by the composer and the optional template.
As this behavior is the same as the one already implemented for message_post, this
feature has been moved into its dedicated method to be called by message_post
or before calling mail_mail.create().
This also allows to have attachments in pure email mass mailing that are attached
to mail.message, not to mail.compose.message model. When unlinking those emails,
the attachments will be also removed, and not lost in the database anymore.

bzr revid: tde@openerp.com-20131126110942-h8p0d9sbjtk0hwlu