The IMAP source now requires a user and password.

The previous implementation allowed for the user and password
to be set to None, in which case the login step would be
skipped. However, this functionality has never been tested
on a real IMAP server supporting PREAUTH. Since it is simple
enough to re-implement if the need arises I have opted to
remove it.
Created pochi.mail.bytes_from_message helper.
Added tests for pochi.mail.inbound.imap.