Fixed mangling of multibyte characters

I went out and fixed the problem specified in the issue I opened earlier today: https://github.com/substack/node-smtp-protocol/issues/31

I replaced manual parsing with the combination of native StringDecoder and a string buffer. My test mails are now parsed correctly.

I'm not sure what's the `self.bytes` part about. Seems to be used solely in tests. I tried not to touch it and left its environment intact.

Haven't touched the tests.