new mailer based on Python's smtplib

This is needed to work on machines without /usr/sbin/sendmail available.
set the From: field based on the user's configuration (user.name, user.email)

This allows having a nice From: header (i.e. name <email>) with the smtplib
mailer. This also allows smtplib to work without setting enveloppesender
(unlike sendmail, Python's smtplib doesn't try to guess the sender's email).
Clean error message on sendmail failure