[MERGE] [ADD] product_email_template new module. It allows ot send template-based emails when validating invoices. This allows to send description, user guides, technical datasheets, ... when selling products.

product_template inheritance: added email_template_id field, link to an optional email.template

invoice inheritance: when validating the invoice, send an email for each product that has a linked email.template.

email_template inheritance: defined a new simplified form view with only attachment_ids and body, to ease the quick edition of email templates. This simplified form view is used for the email_template_id field on the product.template form view.

email_template: when having default_model in default_get, use it to prefetch model_id. Indeed model is a related file, we have to redirect its default to the right model_id field.

bzr revid: tde@openerp.com-20140124120652-vxp0jwf14sl1frme