Adding audit trail fields and triggers to all tables

I need to add the following fields to all tables to existing 8.1 database:

created timestamp without time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,

createdby text DEFAULT CURRENT_USER,

updated timestamp without time zone DEFAULT CURRENT_TIMESTAMP NOT NULL,

updatedby text -- current_user who last updates this record

I need also to add triggers so that updated and updatedby fields ae
automatically set when record is changed.

Where to find script which implements this ?

Andrus.