Change protocol module design:

- Each packet encoder and decoder is in a class
- A packet registry check if there is duplicate packet code type
- Each packet class got a docstring
Next commits will update the NEO source code to use those changes.


git-svn-id: https://svn.erp5.org/repos/neo/trunk@1356 71dcc9de-d417-0410-9af5-da40c76e7ee4