Added a feature suggested by Martin v Loewis, where a new header

Added a feature suggested by Martin v Loewis, where a new header
encoding flag SHORTEST means to return the shortest encoding between
base64 and qp.  This is used for the header_enc for utf-8.  SHORTEST
isn't legal for body_enc.

Also some code cleanup:

- use True/False everywhere
- use == instead of `is' in a few places
- added _unicode() and make consistent the "is unicode" checks
- update docstrings