Updated MailHandler to allow using TLS without having a keyfile/certfile

Hey guys,

Thanks for logbook! After dealing with the pain of python's stdlib logging for years, this library is such a relief! Never want to go back ;)

Attached is a small patch to allow using TLS without explicitly defining keyfile/certfile. I didn't find any unit tests testing this behavior. Also fixed a small typo.

Cheers,
Remco