adds support for smtplib.SMTP_SSL

[milotoor]

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.