Fix for #614 - "Use SMTP encryption" setting sets incorrect value 

Fixed issue #614 - "Use SMTP encryption" setting sets incorrect value.

This now gives the option of selecting 'None', 'TLS' or 'SSL' on the front-end as encryption types which according to http://swiftmailer.org/docs/sending.html#encrypted-smtp are the valid values for encryption type. I've also added a check when the `Swift_SmtpTransport` instance is created in `MailerFactory`; if the selected encryption type is not listed in `stream_get_transports()` it assumes no encryption. This will fix the above error retrospectively for any installations which have `smtp_encryption: 1` in their `config.yml`.