[REF] mail_mail: cool and clean refactoring of the models.

[REF] mail_mail: cool and clean refactoring of the models.
Removed body_html and content_subtype fields. We now entirely
rely on the body field, that holds sanitized HTML.
Added a user_signature field from email_template. mail_mail.send()
is now responsible of adding the signature on emails.
mail_notification.notify() sets the user_signature flag on
emails send for notifciations.
mail_mail.send() is responsible of adding a default subject
on emails send if there is no subject on the mail.message,
and res_id and model are set.
purchase_requisition: removed an override of mail.message
schedule_with_attach.

bzr revid: tde@openerp.com-20120829150002-v8t581oesnw0rhvl