Attack output improvement: better choice of agent names.

When an attack pattern is displayed, Scyther instantiates open variables with concrete names.
This is often more intuitive for humans.

In the case of instantiating role names, we often ended up assigning Alice to the B role and
Bob to the A role. This patch provides a more clever heuristic to find agent names
that start with the same letter as the role (variable) name. In case this fails, we
still try to map Alice to roles starting with 'I' and Bob to 'R', and otherwise we just pick something.

Also added "Simon" and "Pete" to cover some common role names.
Merge branch 'compromisingAdversaries' of github.com:cascremers/scyther into compromisingAdversaries