Use MIMENonMultipart as the base class so that you can't attach() to
these non-multipart message types.

non-us-ascii character sets in headers and bodies.  Some API changes
(with DeprecationWarnings for the old APIs).  Better RFC-compliant
implementations of base64 and quoted-printable.

Updated test cases.  Documentation updates to follow (after I finish
writing them ;).

Text.py and class Text => MIMEText.py and MIMEText

MessageRFC822.py and class MessageRFC822 => MIMEMessage.py and MIMEMessage

These are renamed so as to be more consistent; these are MIME specific
derived classes for when creating the object model out of whole cloth.