unlimited undo/journaling

hello,

i wondered how an unlimited undo feature would be realized in
postgresql.

example:
there is an address book within my application and i want to log
each change some user of my system is doing to an entry. but
when he updates maybe the adress of a person i want to be able
to restore each state which this address ever had as i don't trust
the employees of my fictional company.

so what i want is basically an unlimited undo or journalling feature
for more or less every field.

the idea should be to save the data-difference in some journalling
table, but i'm not sure how this could be done properly. maybe
somebody with some experience can help me here ?

thanks in advance
carl