_handle_text(): If the payload is None, then just return (i.e. don't
	write anything).  Subparts of message/delivery-status types
	will have this property since they are just blocks of headers.

	Also, when raising the TypeError, include the type of the
	payload in the error message.

    _handle_multipart(), _handle_message(): When creating a clone of self,
	pass in our _mangle_from_ and maxheaderlen flags so the clone
	has the same behavior.

    _handle_message_delivery_status(): New method to do the proper
	printing of message/delivery-status type messages.  These have
	to be handled differently than other message/* types because
	their payloads are subparts containing just blocks of headers.

In class DecodedGenerator:

    _dispatch(): Skip over multipart/* messages since we don't care
        about them, and don't want the non-text format to appear in
        the printed results.