Add support for SMTP with SSL.

Fastmail requires SMTP to use SSL but not STARTTLS.  This change enables
ones to turn on SSL without automatically starting TLS.

Here are some relevant links:

http://blog.fastmail.fm/2012/05/21/enforcing-ssltls-encryption-of-all-connections/
https://www.fastmail.fm/docs/encryption.html#ot
use a single `encryption` option to specify STARTTLS or SSL/TLS
add Notifier::Mail#encryption setting (closes #423)

Allows SMTP to be configured to use STARTTLS or directly use a SSL/TLS
connection. Deprecates Notifier::Mail#enable_starttls_auto.