Updated trust update so that it works incrementally, and not like in the
paper (constantly recomputing from the whole history) - that was only to
simplify the formalism. Now, we store trust models and update bit by
bit. The functionality to implement innocent until proven guilty is now
in the trust model class: we give every agent an r bonus at the
outset. If we update trust negatively, and it's the first time (s=1)
then remove the bonus from r.