Differentiate "no-encoded" body (= "8bit", "7bit" and "binary" encoding) and...

Differentiate "no-encoded" body (= "8bit", "7bit" and "binary" encoding) and not-supported endoding by introducing "None" statement in the encoding dict.
Add support for 'quoted-printable' encoding in python 2.3 environments.
Don't modify attachments on edit.
Always save a clean mail body when edit.
Make sure charset declaration and body encoding are consistent.
Refactor API and add lots of usefull methods.
Add some generic methods to edit and get the header and body.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@6332 20353a03-c40f-0410-a6d1-a30d3c3de9de