pipe: Use empty message token '--' if no message

Previously, the subject was duplicated, which is really confusing
pipe: Unwrap forwarded emails

If the mail received into the system has a content type of 'message/rfc822',
then unwrap the inner message and use it as the parsed email. Effectively,
assume the email was forwarded as an message/rfc822 attachment.
pipe: Honor inline bodies as valid message

Previously, if a message had a Content-Disposition header, then it could not
be considered as the body of the message.
fetch: Implement parsing attached email

If the content of the message is of type `message/rfc822`, then support
downloading the body of the message and sending it off to the mail parsing
system, simulating a piped email.
Merge pull request #482 from greezybacon/issue/email-parse-issues

Fixup a few email piping issues

Reviewed-By: Peter Rotich <peter@osticket.com>