[FIX] email_template: when usign the composer in mass mailing mode with

[FIX] email_template: when usign the composer in mass mailing mode with
a template, display the user's signature if asked by the template. The use will
then be able to modify the generated content and to remove / update it if necessary;
Otherwise the signature is not added in mass mailing mode, because the content
of the email is based on the wizard's body, not the template one.

Some rewriting in the onchange: now using a browse instead of a read.

bzr revid: tde@openerp.com-20140423152926-sno0gk8lup9l442p