adds support for smtplib.SMTP_SSL

Certain services require a secure connection to be established over SMTPS rather than use STARTTLS to upgrade a connection made over SMTP.  Python's smtplib provides classes for managing both kinds of connections, but envelopes only has support for SMTP.  I added an extra argument (`smtps`) to the declaration of the `Connection` class (defaulting to `False`), and select between using SMTP and SMTPS based upon its value.