refactor iconv stuff, and normalize message body and headers

refactor iconv stuff, and normalize message body and headers

- Message body and headers are now normalized to the current encoding.
  This should limit the amount of unprintable characters on the screen.
- No more MessageFormatError (was never being thrown anyways).
- Move exception handling to Iconv.easy_decode
- No more Message#convert_from, since Iconv.easy_decode does it all now.