new mailer based on Python's smtplib

This is needed to work on machines without /usr/sbin/sendmail available.

Minor documentation and formatting tweaks added by Michael Haggerty.
Clean error message on sendmail failure
Allow the From: field to be configured separately from envelopesender.

Set the From: field from multimailhook.from, falling back to
(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
envelopesender (unlike sendmail, Python's smtplib doesn't try to guess
the sender's email).

Minor documentation tweaks added by Michael Haggerty.
Merge branch 'smtp-v3'

* Allow emails to be sent using Python's smtplib.

* Provide a clean error message if sendmail fails.

* Set the "From:" address based on the multimailhook.from
  configuration setting, with fallback to (user.name, user.email).

Conflicts:
	git-multimail/README