SMTP reposnse parser rewritten as separate class. + fixes

I had problem with connection to my corporative smtp server. From the wireshark logs I have noticed qxtsmtp doesn't wait at all for response after helo, ehlo etc. I also compared the log with thunderbird's one. It seems thunderbird is more standrards compliant regarding to some other things too.

The qxtsmtp code did't look good for me, so I've written a new parser.

this new code solves my connection problems and also adds some fixes for standards compatibility and reposponse validation