_max_append(): Change the comparison so that the new string is
concatenated if it's less than or equal to the max length.
header_encode(): Allow for maxlinelen == None to mean, don't do any
line splitting. This is because this module is mostly used by higher
level abstractions (Header.py) which already ensures line lengths. We
do this in a cheapo way by setting the max_encoding to some insanely
<100k wink> large number.
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 ;).