trigger on adding / deleting / modify user on pg_authid

Hello,

can I create on PG 9.1 or newer a trigger or something other, that runs a function if a new user is added, changed, deleted to / on the database?
I have got a "usertable" which stores some additional user information (full name, mail address, etc) and I would like to create default data
if a new authid is created. Also I would like to modifiy / delete this rows on the table, if the user is removed / modified.
At the moment I add the new user and create also a dataset on my table

Thanks

Phil