feat(core): Support smtps and STARTTLS for SMTP

Support uri schemes for SMTP, enabling STARTTLS and SMTPS for SMTP
connections.

The new format for configuration value SMTPServer supports passing a
URI scheme (either smtp:// or smtps://). To support old configurations,
if no scheme is given, smtp:// is assumed.

Fixes #31

SoObjects/SOGo/SOGoDefaults.plist

@@ -30,7 +30,7 @@
     SOGoLDAPContactInfoAttribute = "description";
 
     SOGoMailingMechanism = "sendmail";
-    SOGoSMTPServer = "localhost";
+    SOGoSMTPServer = "smtp://localhost";
     SOGoMailSpoolPath = "/var/spool/sogo";
 
     SOGoWebAccessEnabled = YES;