-Modified interface of convertable to add new method that only returns

the authorized formats for conversion.
-Implement mixin for convertable, text_convertable,base_convertable,
html_convertable, metadata_discoverable and document



git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@34090 20353a03-c40f-0410-a6d1-a30d3c3de9de