make strong `SKPSMTPMessageDelegate`'s instance

I always got an error while using the latest version of `skpsmtpmessage`. the error message is `[SKPSMTPMessage setWatchdogTimer:]: message sent to de allocated instance`. The error is showing when skpsmtp try to invoke implemented delegate.

I try to make `SKPSMTPMessageDelegate`'s instance in `SKPSMTPMessage.h` to strong and solve this problem.