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/SOGoDomainDefaults.m

@@ -242,7 +242,15 @@
 
 - (NSString *) smtpServer
 {
-  return [self stringForKey: @"SOGoSMTPServer"];
+  NSString *server;
+  server = [self stringForKey: @"SOGoSMTPServer"];
+  // backwards compatibility
+  if (![server hasPrefix: @"smtp://"] &&
+      ![server hasPrefix: @"smtps://"])
+    {
+      return [NSString stringWithFormat: @"smtp://%@", server];
+    }
+  return server;
 }
 
 - (NSString *) smtpAuthenticationType